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 ...
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 ...
In 5e you don't disengage "from" someone. Instead, you just don't provoke OAs when you move:
If you take the Disengage action, your Movement doesn’t provoke Opportunity Attacks for the rest of the turn.
You definitely can take the Disengage action if you don't see any enemies. Moreover, you can Disengage without enemies at all. The only requirement ...
The way you approach this depends on how your players interact with you as the DM. I've used two methods that seem to work.
If your players are rules-oriented, new to the game, or if you haven't played with them long enough that they trust you to not make things happen "just because," what you're asking is difficult.
A player who ...
The Sentinel feat
A quick search for the keyword "Disengage" across all published classes returns no class features that allow a player to make an opportunity attack against an opponent that has taken the Disengage action, or otherwise specifically cancel a Disengage.
However, there is the Sentinel feat (PHB, p. 169-170), which grants the following benefit ...
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.
Yes, Fancy Footwork applies in these instances
Your reading is correct. To trigger Fancy Footwork all you need to do is:
make a melee attack against a creature
The cantrips Booming Blade and Green-Flame Blade use identical wording when they say:
you must make a melee attack with a weapon against one creature
So both Booming Blade and Green-Flame Blade do ...
Unless the source of your advantage is dependent on something that doesn't apply to attacks of opportunity (like taking the "Attack Action"), you can absolutely get advantage on an attack of opportunity.
In the case of your specific example, the Mounted Combatant Feat states that (PHB, p. 168):
You have advantage on melee attack rolls against any ...
This does not work.
Your second opportunity attack is not an opportunity attack, rather, you are using your reaction per this clause from Tunnel Fighter:
you can use your reaction to make a melee attack against a creature that moves more than 5 feet while within your reach.
Notice "opportunity attack" is absent from this portion of the fighting ...
Goblins can avoid opportunity attacks on their turn, but Dissonant Whispers should help.
If you take the Disengage action, your movement doesn't provoke opportunity attacks for the rest of the turn.
If goblins use their bonus action to Disengage every turn, you cannot even attempt to hit them (as there is no trigger to use your reaction)....
The booming blade cantrip discourages movement, and is unaffected by Disengage.
As well as the Sentinel feat, which allows a character to make opportunity attacks even if the enemy disengaged, the booming blade cantrip published in the Sword Coast Adventurer's Guide (p. 142) performs a similar role.
When you cast booming blade, you make a single melee ...
You can't use the bonus action attack after an opportunity attack.
The extra butt-strike attack that you can make with the Polearm Mastery feat has several requirements that you can't meet in this situation. Lets look at the relevant part of the Feat's rules (from the PHB, page 168):
When you take the Attack action and attack with only a glaive, halberd, ...
Yes, the OA triggers, but they attack at disadvantage.
The moving away still triggers an opportunity attack ...
You can make an opportunity attack when a hostile creature that you
can see moves out of your reach. To make the opportunity attack, you
use your reaction to make one melee attack against the provoking
The description of the Dodge action states:
Until the start of your next turn, any attack roll made against you has disadvantage if you can see the attacker, and you make Dexterity saving throws with advantage.
Opportunity attacks require attack rolls, so they would all have disadvantage, as long as the attackers were not invisible.
That being said, I ...
Player handbook section on Opportunity Attacks says (pg 195):
You also don't provoke an opportunity attack when you teleport or when someone or something moves you without using your movement, action or reaction. For example, you don't provoke an opportunity attack if an explosion hurls you out of a foe's reach or if gravity causes you to fall past an ...
You flank when you are adjacent to an enemy:
Flanking on Squares. When a creature and at least one of its allies are adjacent to an enemy and on opposite sides or corners of the enemy's space, they flank that enemy
You are adjacent when you are within reach. You are within reach in the moment of the OA:
The attack interrupts the provoking ...
Polearm Master reads as follows (emphasis mine):
"When you take the Attack action and attack ..."
Since opportunity attacks are not "taking the attack action", Polearm Master does not apply. You make only the regular attack as your opportunity attack.
You can disengage as an action on any turn whether you can see the other creature or not. Keep in mind that if the creature cannot see you because of the darkness then they wouldn't get OA anyway. However, if the creature can see in magical darkness then they would still get the OA unless you disengage.
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 ...
As you've noted, Colossus Slayer limits to only once per turn. If the opportunity attack occurs off your turn, then you can use it again because you are not using it 'on your turn.'
Do note that you still only get one Reaction, so you effectively you can really just do this twice per round (once on your turn, once off your turn with a reaction spent.)
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 ...
Cast a Spell does not inherently trigger Attacks of Opportunity.
Cast a Spell, on its own, is not a Manipulate action (it does not have the manipulate trait), it is not inherently a ranged attack, it is not a move action (it does not have the move trait), therefore it does not provoke attacks of opportunity on its own.
But Components can change that
The rule states:
You can make an opportunity attack when a hostile creature that you can see moves out of your reach.
The Giant reach is unknown (see the related question), but the one and only melee attack from the stats block has 15 ft reach. If we assume its reach is 15 ft, and you are moving just 5 ft away from it, you are not "out of its reach".
Spells and Items
You will exclude a few edge cases, though it is arguable whether or not these could be used in an opportunity attack anyway.
The dismissal option of Dispel Evil and Good.
Using Flame Blade, Mordenkainen's Sword, or Vampiric Touch to attack.
This will have a major effect on some monsters, who will be unable to make ...
No, the range of the spells stays the same
The description of the War Caster feat (PHB, 170) says:
When a hostile creature's movement provokes an opportunity attack from you, you can use your reaction to cast a spell at the creature, instead of making an opportunity attack. The spell must have a casting time of only 1 action and must target only that ...
No, you can't.
Your reasoning is correct. Activating the Tentacle Rod isn't an attack, so it can't be an opportunity attack. (The Rod itself makes attacks, but only when you activate it--it doesn't get its own turns, so it can't take reactions of its own.)
If you really want to use it reactively, you can ready an action to activate it, but that's at the ...
Opportunity attacks happen when the opponent leaves your reach
Standing up doesn’t do this. Nor does running around you in circles within 5-feet. The target must move from within 5-feet of you to not within 5-feet of you.
If they are prone while they do that (i.e. they crawl) you can use Hooves.
By RAW, No
The language there is clear, you do not provoke opportunity attacks after you've made a melee attack. Full stop. It's not a "can choose not to provoke", it's just that you don't provoke them.
As an aside about hard choices, I had a paladin with the sentinel feat, which sets the movement of a creature to 0 when I hit them with an ...