If I cause an opponent to leave my reach with an ability, spell, or effect can I use my reaction to take an Opportunity Attack against them?
Yes, but only if the ability forces the enemy to move out of reach using their movement, action, or reaction
The rules state that only when the enemy uses their movement, action, or reaction to move out of your reach will they provoke an Opportunity Attack from you.
From the PHB:
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 enemy. (PHB 195)
And from the Sage Advice Compendium:
A creature doesn’t provoke an opportunity attack if it is moved without the use of its movement, its action, or its reaction. For example, the effect of the antipathy/sympathy spell requires the target to use its movement, meaning that it would provoke opportunity attacks when it does so.
Similarly, dissonant whispers requires the target to move using its reaction (if available), so that activity also provokes opportunity attacks. In contrast, a creature that’s pushed by a gust of wind spell does not provoke opportunity attacks.
So, the answer to the question will depend on the whether the ability you are using to force the opponent to move uses their own movement, action, or reaction to do so. If it does, then they provoke an Opportunity Attack, if it doesn't it doesn't.
Gust of wind vs dissonant whispers
Looking at the examples given in the text above we can see that dissonant whispers definitely qualifies for provoking an Opportunity Attack since it specifies that the target must use their reaction to move:
On a failed save, it takes 3d6 psychic damage and must immediately use its reaction , if available, to move as far as its speed allows away from you.
Whereas gust of wind moves the target but without using any of the target's movement, action, or reaction:
Each creature that starts its turn in the line must succeed on a Strength saving throw or be pushed 15 feet away from you in a direction following the line.
Thus, the movement from dissonant whispers provokes an Opportunity Attack but the movement from gust of wind does not.
Examples of forced movement that does not provoke Opportunity Attacks
None of these spells or abilities force the enemy to use their own movement, action, or reaction to leave your reach and thus do not provoke opportunity attacks. These are only examples and not a complete list.
- Thorn whip
- Lightning lure
- Gust of wind
- Repelling blast
- Thunderbolt Strike (Tempest Cleric)
- Open Hand Technique (Monk)
- Shove (and here)
- Moving a grappled creature
- Falling or getting thrown by environmental effects (gravity, explosions, etc.)
Examples of forced movement that does provoke Opportunity Attacks
These spells or abilities force the enemy to use their own movement, action, or reaction to leave your reach and thus do provoke opportunity attacks. These are only examples and not a complete list.
- Command ("Flee")
- Dissonant whispers
- Turn Undead
- Goblin Boss's Redirect Attack ability
Polearm Master and Sentinel do not affect how forced movement works with OAs
Two commonly used feats that modify the rules for Opportunity Attacks are Polearm Master and Sentinel. However, neither feat changes the rule regarding forced movement not triggering Opportunity Attacks.
Sentinel only overrides a creature's ability to use disengage to prevent Opportunity Attacks:
Creatures within 5 feet of you provoke opportunity attacks from you even if they take the Disengage action before leaving your reach.
And Polearm Master only allows one to make an Opportunity Attack when a creature enters your reach in addition to leaving your reach.
While you are wielding a glaive, Halberd, pike, or quarterstaff, other creatures provoke an opportunity attack when they enter your reach.
Since neither ability says anything about allowing Opportunity Attacks to be provoked on movement that does not use an enemy's actions, reaction, or movement then the general rule still applies.
Note that the movement doesn't have to be willing it just has to use their movement, action, or reaction. There are some effect that trigger on willing movement, but that is not relavent here (see Would a frightened, fleeing creature be considered moving willingly? for more discussion on this).
This Q&A discussing the balance of houseruling that forced movement provokes even if the enemy does not use movement, action, or reaction might also be of interest.