What Provokes AoO in 5e

[Nosta]

I'm working on a Polearm Master + sentinel + Tunnel fighter build and I like to know what all provokes AoO from me .

What should a Battle look like if I'm able to to take full advantage of my build

Battle Master fighter

[Rara1212]

I'm working on a Polearm Master + sentinel + Tunnel fighter build and I like to know what all provokes AoO from me .

What should a Battle look like if I'm able to to take full advantage of my build

Battle Master fighter

AoO: Moving out of your reach, (PAM) trying to move into your reach. These ones won't cost your reaction while in Tunnel fighting "stance"
Reaction attack(not AoO): (Sentinel)enemy withing 5ft attacking ally, (Tunnel fighter) Enemy moving 5ft inside your reach. These ones will cost your reaction.

Depending on battle you either want a reach polearm, or a Quarterstaff/spear + shield. As AoO trigger right before the creature exits(or enters) your reach, sentinel allows you to lock down an enemy inside your reach, our outside.

In an enclosed space(15ft wide or smaller) you probably want 1h + shield, for more AC, as you have a smaller are you need to cover.
In open spaces you can use a reach polearm to catch more enemies in your reach. If there are a lot of enemies, you can lock down 24 enemies, and only 8 are able to attack your.



If the enemy is melee based(and doesn't have reach), you can take the reach polearm, stand next to your casters(have them within 5ft of you) and the enemy won't ever be able to come close(until you miss an attack, tho then you can shove them away)
The other option is to lock them close to you, with sentinel.

[Keravath]

Without class or feat features, the only thing that triggers an opportunity attack is a creature moving out of your reach. An opportunity attack also usually uses your reaction.

Polearm master lets you make an opportunity attack when a creature enters your reach.
Sentinel will set the speed of a creature to zero if you hit it with an opportunity attack. In addition, creatures leaving your reach will provoke opportunity attacks even if they take the disengage action.

Tunnel fighter allows you to enter a defensive stance as a bonus action which then allows you to make opportunity attacks without them requiring your reaction.

However, if you use your bonus action to enter a defensive stance you don't get the bonus action attack provided by PAM.

Anyway - the combination allows you to make an unlimited number of opportunity attacks at creatures that enter and leave your reach. If you hit them then their speed will drop to zero.

It is probably most useful in the first round of combat when melee creatures approach you to attack. Once they are in place, it becomes a trade off between the bonus action attack from PAM or the defensive stance which doesn't help unless there are more reinforcements or creatures running away which often isn't the case.

Also, the tunnel fighter fighting style is from unearthed arcana so you may want to check with your DM if they will allow it. Some folks consider it unbalanced.

[da newt]

A bugbear might be fun for this build as you get an extra 5' reach on all attacks during your turn.

Echo Knight Fighter and Ancestral Guardian Barbarian both add some interesting things to this build as well.

[MaxWilson]

Also Mage Slayer can give you a reaction attack against spellcasting within 5'. It's particularly good on monks because Stunning Strike. However, it's the DM's call whether the reaction attack happens before or after the spell takes effect: is the trigger "creature casting" or "creature having already cast"? It's much better if the Stunning Strike can actually prevent the spell from going off.