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Opportunity attacks have a long history in the world of tabletop games and D&D in particular. 3.5e/PF both had an exhaustive list of actions which can or can not trigger an OA.

Various games have their own OA mechanics, but one trigger always remains the same.

  • 5th edition simplified things a lot. Only the one single trigger left:

    You can make an opportunity attack when a hostile creature that you can see moves out of your reach.

  • Starfinder also reduces the list of AoO triggers greatly, comparing to its ancestor, to three points only, and "an enemy goes away" trigger is still there:

    When you threaten a space and the opponent moves out of that space in any way other than a guarded step

  • Open Legend, a game influenced by but not directly derived from D&D, has the same only one trigger:

    If you are wielding a melee weapon, and an enemy moves from a space within your reach to a space that is not within your reach, you may make a free attack against the enemy.

It seems the game developers consider this particular trigger as the most important one. To my knowledge, many wargames uses the similar rule. So what happens if we remove it? Why do we need it, in the first place?

Is there a substantial problem with turn-based gameplay, which OAs solve?

The reason I ask is because there are 5e-based games which do not have OAs at all (Five Torches Deep, for instance). I want to know what changes I should expect within basic 5e gameplay (no feats, no variant rules) if the DM introduces a "no opportunity attacks" house rule.

I'm more interested in base mechanics, rather than particular OA-dependent 5e spells or features (like Rogue's Cunning Action becomes less useful, etc.)

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Coming from a D&D point of view...

The game becomes a lot more mobile, speed is more valuable, the concept of front-liner diminishes, and being at range is either difficult or not important.

I tried this for a one-shot once with a couple of new players, and we removed OAs for simplicity's sake. The more experienced players at the table had some fun exploring the consequences of this.

Mobility

Everyone runs in, attacks, runs away. Both enemies and party members take advantage of cover and we noticed that the disposition of units in the map was much more chaotic.

Speed is more valuable

If mobility is heightened, then speed will improve accordingly. Both the Tabaxi and the Monk appreciated jumping into backlines without hassle and focusing on the squishy ranged goblins that were harassing them.

Frontliners can't frontline as well

One of the reasons a Barbarian or a Fighter stands in front of the party and prevents multiple enemies from going near the Wizards and Sorcerers in the back is their menacing presence and the threat of retaliation. Without AoO, enemies could easily run around and jump into backlines, and the party's melee frontliners just did the same.

Being at a range is more difficult, but at the same time, possibly unnecessary

In closed off areas, where you can't be shooting your bow from 150 ft away, enemies quickly get the jump on ranged attackers, and new tactics are required. If the character is at a range because it does more damage (Dex Fighter with Sharpshooter feat), then they can just step 5 ft from any enemy and attack. If they are at a range because they are very squishy (Wizard with 0 CON modifier), they require new tactics to always be at a distance.

Cat and Mouse

While it didn't happen to us, in open areas, a game of cat and mouse can arise. Goblin dashes to Wizard. Wizard Dashes away. Gobling Dashes to Wizard. Repeat. Because there is no consequence for running away, kiting enemies is easier (you don't take damage for running) in wide spaces.

Overall, it was a fun experience for players, but we decided it didn't work for us. The players enjoyed the fact that, originally, their front-liners were there preventing enemies from running through them. They didn't enjoy enemies now running away and kiting them. But to each their own.

Is there a substantial problem with turn-based gameplay, which OAs solve?

I would say it depends on your expectation. I've only ever played D&D 5e, and I expect our melee combatants to have some ways of preventing enemies from running past them. OAs are one way (a big one, as every character has access to them, from Barbarians to pets). If you don't have such an expectation, I don't see it becoming a big issue. You will simply build your ranged characters expecting to have to handle melee enemies often. All that being said, this is a mere opinion, and to each his own.

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  • \$\begingroup\$ Doesn't it make pursuing an enemy completely useless ? \$\endgroup\$ – MakorDal Apr 6 at 13:32
  • \$\begingroup\$ @MakorDal Not completely. If they are within your reach, you can still attack. If not, you can run behind them with a Dash to surround them (if they run, the distance on the next turn you need to cover might be smaller). You have ranged attacks, Dashes as bonus actions, etc. \$\endgroup\$ – BlueMoon93 Apr 6 at 13:33
  • \$\begingroup\$ @MakorDal there are option chase rules in the DMG for actually fleeing that make it different than extending combat rules. I've found those rules make a lot more sense to play fleeing or chase sequences. \$\endgroup\$ – GcL Apr 6 at 13:36
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    \$\begingroup\$ @MakorDal depends on how do you define "useless". Sometimes the goal of a combat is driving enemies away. When you force an enemy to retreat you achieve this goal. Anyways, no one can run forever. \$\endgroup\$ – enkryptor Apr 6 at 13:57
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    \$\begingroup\$ To help address the "Frontliners can't frontline as well" issue, Pathfinder 2e makes AoO a feature available to certain classes. Haven't played it personally, but I hear it adds mobility like what you experienced while still preventing the frontline from sprinting into the squishy backline. Assuming good party composition and tactics, at least. \$\endgroup\$ – Carl Kevinson Apr 6 at 18:51
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The Opportunity Attack mechanic supports control of space

Without it, if characters act only on their own turns, it's very difficult to stop characters moving to places where you don't want them to get to, and if there's a significant movement advantage it might even result in a character being unable to interact with someone who moves past them. Readied action and delay mechanics help with that last part, but opportunity attacks definitely contribute to controlling space around you.

To help illustrate this, I'll share a couple of examples of other ways systems have addressed this problem.

The clearest example I can think of is Valor: The Heroic Roleplay System, which is very much built for the kind of tactical combat play this question addresses. While it's possible to build a character with an equivalent to an attack of opportunity, the default is that movement within an enemy character's reach costs double. This makes for a comparable dynamic - even without specifically investing in a tank-style toolkit, frontline melee characters can engage the enemy to make it difficult for them to run past and engage less resilient characters behind them - but it's more deterministic. A basic opportunity attack in D&D5e doesn't stop the enemy moving if they decide it's worth getting hit; the Valor zone of control mechanic might make it absolutely impossible to reach a position that would be reachable without it, but might have no impact at all if the enemy only wanted to move half their speed anyway.

Exalted 3rd Edition is another illustrative example. It uses an abstract range band mechanic rather than a battle grid. In this system, characters can't move away from enemies in close range unless they use their action for the round to disengage, which also requires an opposed roll against all enemies they're engaged with - evasive characters find it easier to escape combat, but more mobile foes find it easier to prevent that. As with Valor, it's about restricting movement rather than punishing it - a failed Disengage roll means the character can't move at all.

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    \$\begingroup\$ This answers the title question very well - why are OA's needed, why is that mechanic in there. \$\endgroup\$ – KorvinStarmast Apr 7 at 14:21
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Pathfinder 2nd Edition: The Middle Road

I don't have at-the-table experience with PF 2e, so take this with a grain of salt.

In Pathfinder 2e, Attacks of Opportunity (AoOs) still exist, but are much rarer. Rather than a default action that any character can perform, only certain classes can make AoOs. Fighters get AoOs as a class feature at level one, a few other martial classes (Barbarian, Champion) can choose to get it at 6, and any character multiclassing into Fighter (i.e. taking the Fighter Archetype) can choose it at some level. This considers the Core Rulebook only.

I believe the goal is gain the advantages described in @BlueMoon93's answer while avoiding the disadvantages. Characters will be more willing to move more than a 5-foot step, but it is still possible for a martial character to lock down an area like they could in PF 1e, but they need to be built to do so. The result should be battles where characters move more often, but without enemies breezing past the front line. Again, this is theoretical as I haven't played PF 2e combat.

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    \$\begingroup\$ I agree with your assesment. AoOs are too important for the Tanking/Frontline role. But also too abundant in everyone else. They are trying to go the middle road | It is worth noting that this will hit the NPC propably harder then the Player Characters. There should be not too many Monsters that can do AoO's by default. I see nothing like it on the Hobgoblin, for example (the monster I would most expect to have this): 2e.aonprd.com/Monsters.aspx \$\endgroup\$ – Christopher Apr 6 at 22:37
  • \$\begingroup\$ @Christopher you say "AoOs are too important for the Tanking/Frontline role", could you please elaborate this into an answer? Why AoO are so important and why tanking becomes doubtful without them. That's exactly what the question is about. \$\endgroup\$ – enkryptor Apr 7 at 14:06
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    \$\begingroup\$ @enkryptor see BlueMoon93's answer. \$\endgroup\$ – Christopher Apr 7 at 16:53
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In the same way as Carl Kevinson's answer, a comparison to another D&D variant might help. In this case 4E. They tried a odd mix of expanding and reducing it at the same time.

  • they did keep the 3.X rule and indeed expanded it - everyone get's to make one OA on every other creatures turn (no more 1/turn limit).
  • Range on attacks no longer automatically increases your OA threat range. For that you need "Threatening Range".
  • you can only use melee basic attacks for OA's. A handful of abilities "can be used as melee basic attack" or are special "opportunity actions", so you can use them there. But generally those are rare. So this attack will be comparatively simple/weak: attack vs AC, plain weapon damage, no secondary effects
  • shifting is a thing. The shift move action is basically just the good old 5-feet step: you move 5 feet, you do not provoke a OA. But there are a lot of abilities that allow you to shift farther then that. The Monk Power list is full of At-Will powers that ignore those OA because of long-range shifting (Crashing Wave, Dragons Tail, Five Storms) or at least disincentivize/penalize them (Blistering Finish, Dancing Cobra, Fallen Needle).
  • shifting is such an issue, that they had to give the tank classes special ways to deal with enemies running away. The fighters "Combat Challenge" and the Berserker's "Vengeful Guardian" explicitly trigger on shift-moves. While most others retaliation simply have no/very little range constraints.

Comparatively, Pathfinder 2E's solution is a lot simpler. As I learned the hard way from both RPG systems and languages: "It is never the rules that are the problem - it is always the exceptions.

OA are a solution to a core problem of the rule system. A simple "No more OAs" might cause more problems then it helps (see BlueMoon93's answer). And actually removing them at design time is involved, as you can see with the Pathfinder 2E information.

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  • \$\begingroup\$ This does not answer, why do we need AoOs at all. Could you please elaborate "A simple "No more OA's" might cause more problems then it helps."? \$\endgroup\$ – enkryptor Apr 7 at 12:47
  • \$\begingroup\$ @enkryptor BlueMoon93 already elaborated on the downsides of "no OA's" in all nessesary detail. My goal was only to add a 4E perspective, as how it plays out can be hard to grasp if you never actually played it. \$\endgroup\$ – Christopher Apr 7 at 16:51
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Monsters suffer more than players, so do Warcasters and Rogues

This answer is based entirely on 5e, where you only ever get a single opportunity attack per creature, because that is what you are thinking of running. In games where you can attack many times, this answer's logic will not work.

I've DMed plenty of games in 5e, and I've come to the realization that opportunity attacks really are not as scary to monsters as people make them out to be.

You might argue that a guy with a shield in plate armor is protecting the backline by threatening an opportunity attack if enemies try to run past him, but in reality, for most enemies the tactical choice will be to take the opportunity attack and run past to get to the interesting targets.

A warrior standing there and threatening to hit for 10-ish damage once if they run past is not going to make up for the amount of damage they are going to be taking if they pointlessly wail on the wrong target with their attacks. In my latest campaign, the "tank" is essentially a walking block of armor class. Most monsters need a critical hit to even hit him, because the player stacked as many armor class increasing effects as they could find. Most smart enemies just run straight past him, take a bit of damage and start attacking the people who're actually worthwhile to attack.

However, players tend to be drastically outnumbered. That 10 damage the monster might take from provoking an opportunity attack might be 40+ damage for a player running through an undead horde to get to the Necromancer controlling them.

As a result, monsters get very little benefit from this change, while players get a lot. In addition, some creatures might be more affected than others. The Hydra comes to mind, its Reactive Heads ability doesn't do anything besides granting extra opportunity attacks.

There are, however, exceptions, where players are harder hit than monsters:

Rogues

Rogues can sneak attack a second time in a round, once in their own turn, and once again on their opportunity attack. A rogue threatening an opportunity attack with sneak attack damage is a very serious threat, because you're suddenly looking at a lot more damage than a single hit would normally deal.

Warcasters

You mention that you won't be using feats, so this isn't important for you, but Warcasters can deal a lot more damage with their opportunity attack than most melee classes will be able to do. Special mention goes to Eldritch Knights with Booming Blade, which is one of the few classes I find that will reliable prevent big monsters from simply ignoring opportunity attacks, because the amount of damage they will take from moving, then taking another Booming Blade to the face and then moving again is insane.

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    \$\begingroup\$ This is not exactly what I was asking.. It is obvious that OA-based tactics would suffer without OAs. This very tactics exist because of OAs, it's just a further development of the idea. What I'm trying to grasp is this idea itself. OAs themselves exist because of... what? \$\endgroup\$ – enkryptor Apr 7 at 16:39
  • \$\begingroup\$ @enkryptor My bad, I completely misunderstood your question then. \$\endgroup\$ – Theik Apr 7 at 17:52
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This is a question I've thought about a lot because I'm making my own d20 game with the aim of keeping rules as light as possible. To that end I've done away with OAs. My play testers have responded positively because they are more mobile on the battlefield.

Play Expeerience

With OAs in force, positioning tactics stop after round 1 or 2 because after that everyone is locked in place on the spot and melee becomes a slog.

Without them, they can reposition more easily, switch up their tactics if they're failing, or even retreat if things are going badly without fear of a TPK.

Fighting is more fluid and keeps you paying attention while waiting for your turn. This makes for very 'cinematic' fights where opponents dance around trees, chase through buildings, jump on tables. I compare typical D&D fights to Rock 'em, Sock 'em Robots with dice, where as the games I'm testing feel more like dynamic movie fights.

As for melee fighters protecting ranged fighters, I find that ranged fighters do just fine. They tend to maximise Dexterity which gives them high AC. They also do more hiding and think more tactically themselves, rather than leaving that to the meatheads at the front. Together with carefully chosen healing options and other options like the Tough or Mobile feat, they don't do so badly. This might prevent them from taking things which would enhance their attacks, which is a trade off which that I'll come to dislike later in this post, but I think it's less severe here.

OAs exist because...

As for why OAs exists at all, my guess has always been that it's supposed to be a simulation of something. I think the idea is that if you turn your back to run then you expose yourself to attack. I think this is wrong. Consider that ALL self defence advice begins with 'run away'. I think that the Disengage action is easier than the designers think and should be free, where as the OA action requires much more whit and agility and should be the action that requires the learning of a new skill.

Other Thoughts

From a game design perspective, starting with this false notion that fleeing is hard creates a rules arms race to help players Not Suck, rather than Be Awesome. It just creates more default rules to read and remember and bloats out Feat and Class Feature options with stuff that is just there to overcome a problem that you created, not build on the cool stuff you can already do. If OAs exist at all it should be as a bonus, not a penalty. (And don't get me started on shooting into combat!).

Theik also makes the great point that suffering one or two OAs is not a big deal for any melee creature above about level 3, so why not suffer it and do what you like anyway!? I think that the aversion to incurring OAs is more psychological than tactical. This psychological deterrent against running wherever you want seems to be the only real value I can see, but it's value crumbles when you realise the dull static combat that it encourages. I said earlier that ranged players might have to take some options to buff their defences, I don't think these fall into the Options to Not Suck category, but rather make the player feel more powerful than not-weak.

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  • \$\begingroup\$ "why OAs exists at all, my guess has always been that it's supposed to be a simulation of something" — I honestly don't think this is the case. Many computer tactical games implement some kind of OAs. There must be the reason for that, aside from "it looks plausible". \$\endgroup\$ – enkryptor Apr 10 at 11:15
  • \$\begingroup\$ I'm open to suggestions. The only other reason I can think of is "it breaks the game if you don't have it" which seems even less satisfying. \$\endgroup\$ – Benjamin Connell Apr 11 at 16:50
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If we remove opportunity attacks from the game, the following scenarios can occur:

  • If two creatures are the same speed, and only have melee, one can keep away from the other guaranteed with no chance for the other to continue attacking them.
  • Melee rogues can now kite any creature they are faster than with their bonus action dash. The creature's only option is to ready a melee attack, which drastically reduces their output if they have multiple attacks by default.
  • Enemies now have no reason not to run straight past your front line Barbarian/Paladin and dog pile onto your squishy Wizard/Sorcerer.
  • Enemies and players alike have zero reason to fight the person in front of them. A little opinion leaking in here, but this kills the cinematic aspect of melee combat. Duels never really happen. After all, why would a barbarian ever attack the opposing barbarian when they could just hit the wizard?

The main consequence here is that front-line no longer serves a purpose unless they can physically block a passageway, or grapple. Monsters with low movement speed (gelatinous cubes, for example) become useless, and difficult terrain becomes a HUGE deal. If you're melee without high movement speed in this scenario, you're pretty much dead in the water (especially if you're actually in the water, and don't have a swimming speed).

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