Would there be balance issues if I allowed opportunity attacks against any creature, not just hostile ones?

Question

As shown in this question and answer, I am confused by the terms "hostile" and "enemy". So I was wondering if in my own game I could simply remove the "hostile" from the opportunity attack rules, because I think there it is particularly useless.

Currently, the rule on opportunity attacks says (PHB, p. 195; emphasis mine):

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

I'm thinking attacks are allowed against allies; why shouldn't attacks of opportunity be?

Would there be balance issues if I house-ruled that opportunity attacks can be made against any creature, not just hostile ones? Are there any spells, effects or others that now unexpectedly work differently?

The revised rule would read:

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

Edit: Based on one of the answers about the warcaster, I can see why it shouldn't be like this. Now one of the reasons to allow one to attack an ally would be for example when you get turned against them, however they are not hostile towards you. So a solution should allow AoO against enemies that are not hostile towards you, but it shouldn't be possible if you don't actually mean to harm the enemy.

Answer 1

This Slightly Increases the Power of the Players

This is one of those rule changes that doesn't directly increase power level but instead increases the options available to the player. Fundamentally it should be balanced because NPCs and PCs can both use it, however it is the sort of thing players are far more likely be take advantage of than monsters.

A couple of examples where players could use this to their advantage:

War Caster Feat

As Patron Paton excellently points out, the War Caster feat has the benefit:

When a hostile creature's movement provokes an opportunity attack from you, you can use your reaction to cast a spell at the creature, rather than making an opportunity attack. The spell must have a casting time of 1 action and must target only that creature.

Your change would allow buff and healing spells to be cast on allies as they leave your reach. This significantly improves the action economy for support casters and opens up a lot of new combat combinations.

Raging Barbarians

AuxTaco points out a case where this rule could be used to keep your barbarian raging. The rage feature requires you to attack or take damage since your last turn or your rage ends. This modification would allow the barbarian to run past an ally and take damage thus continuing their rage, or alternatively the barbarian could attack an ally with their AoO to achieve the same result.

This is a somewhat meta-gamey option but it is a valid option per the rules with your modification. Perhaps situational and unlikely to make a major difference overall, but still a net positive for player options.

Other niche combinations

The two situations above are just examples of the kind of trickery you can pull off with this rule change, there are likely many more.

I can think of a strange combinations of a character who gains a benefit from reducing a creature to 0 HP and a death domain cleric who heals more effectively from 0 HP. Combining these abilities to knock down and then heal an ally could provide a net benefit to the party overall.

Only creatures you are hostile to

Based on some comments/edits you made and Admiral Jota's answer I understand you have recognised the issues with this rule and instead are considering a smaller modification.

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

The problem with this in my mind is that the players determine who they are hostile to and can easily exploit this. "I am hostile to the rogue, he stole my orange juice this morning". A good DM can easily deal with this but it would be better not to leave it open to interpretation.

Creatures you mean to harm

Your other "mean to harm" suggestion would have a rule something like this:

You can make an opportunity attack when a creature that you can see moves out of your range, provide you mean them harm.

This fixes some of the issues with the "that you are hostile to" variant but does open up some new ones. What counts as "mean them harm"? Some spells cast with War Caster deal no direct damage but are valid options.

Conclusion

Overall this house-rule isn't particularly unbalanced but does open up some interesting combinations. As most of these combinations actively reward intelligent play from the party I don't really consider that a bad thing. I suggest you allow this on a play-test basis and be prepare to modify/remove it should it cause issues.

Answer 2

From memory the War Caster feat is the only thing that might work well with this. Its third benefit is:

"When a hostile creature's movement provokes an opportunity attack from you, you can use your reaction to cast a spell at the creature, rather than making an opportunity attack. The spell must have a casting time of 1 action and must target only that creature."

Assuming you also change the 'hostile' in this wording, this means a spell caster could cast a one action buffing/healing spell against any ally that leaves their melee reach.

Answer 3

A very specific true story where this would have helped:

The party enters a large room with visible enemies at the far end and a side hallway 80 feet from the entrance. The (small) PC monk wins initiative and Dashes forward. At the end of his move, he becomes aware of a large number of archers waiting in the side hallway for their turn to come up.

The barbarian is next. With his 40' speed, he can Dash to and block the side hallway, but would really prefer not to be peppered with arrows. He can't Rage to resist the damage; it would wear off immediately:

Your rage lasts for 1 minute. It ends early if you are knocked unconscious or if your turn ends and you haven’t attacked a hostile creature since your last turn or taken damage since then. You can also end your rage on your turn as a bonus action.

With your revised rule, he could Rage, Dash by the monk (who punches him in the arm as he runs by), block the hallway, and resist the coming onslaught.

Answer 4

There would be "altered play" but not necessarily balance issues.

With most rules, they are written in the best possible fashion by the developers in order to promote balance and fairness. However, it is even indicated within the Player's Handbook and Dungeon Master's Guide that these rules should be a guideline, with ample opportunity to tweak them to any way you deem fit.

Since this rule would apply across ALL beings in your campaign(s), it is not exactly a "balance" issue, seeing as all creatures are affected by this equally, therefore preserving balance. It would, however, create some crazy, and perhaps convoluted fighting scenarios, though you aren't being forced to utilize your reaction in the form of an opportunity attack in the first place, again, mitigating the impact this house-rule would have on the overall flow of battle/the campaign.

As far as affecting certain features, spells, etc., most of these things are worded in a way where your attacks are still limited to specific targets. If you wish to then extend this house rule to these situations, you may experience some strange circumstances, but again, it would all be still be balanced since enemies would undergo the same changes.

A creature can be hostile without being an enemy.

While not a direct answer to your question, as it is all up to interpretation, type of campaign you are running, how combat heavy it is, along with many other things, you can attack a friendly or neutral PC or NPC with an opportunity attack under the correct (and very simple) circumstances.

In my own form of RAI: The difference between Hostile and Enemy is that an enemy is counter to your alignment, goals, or other way of virtue that inhibits it from being your ally or even neutral toward you. A hostile creature (or other being) is something that means to do you harm, which can definitely be a friendly who is intending to cause you harm (even if in a playful manner).

However, RAW indicates for "Hostile" (DMG Pg. 244):

A hostile creature opposes the adventurers and their goals but doesn't necessarily attack them on sight. For example, a condescending noble might wish to see a group of upstart adventurers fail so as to keep them from becoming rivals for the king's attention, thwarting them with slander and scheming rather than direct threats and violence. The adventurers need to succeed on one or more challenging Charisma checks to convince a hostile creature to do anything on their behalf. That said, a hostile creature might be so ill disposed toward the party that no Charisma check can improve its attitude, in which case any attempt to sway it through diplomacy fails automatically.

Unfortunately, there is no exact definition on what makes an "enemy" an "enemy", so feel free to adopt my own interpretation from above or develop your own beyond what is indicated in my answer.

Answer 5

Based on this comment you made:

I would like for the players to be able to make an attack of opportunity against any creature they want. An unexpected side effect would lead to cases where they now make an AoO even though generally they do not mean to harm the creature.

In order to achieve this while limiting the the unexpected side effects, you could choose to change the wording of the rule to:

You can make an opportunity attack when a creature you're hostile to that you can see moves out of your range.

I believe this should still be within the RAI, but would permit characters to use opportunity attacks without needing to rely on the mental or emotional state of the target. This is also more similar to the original wording than your initial proposal, which should reduce the unintended side effects.