Opportunity Attacks: react before the target leaves reach

SRD 5.2.1 · Combat

At the table

1. Check that you can see the creature and that it is leaving your reach using a listed action or speed.
2. Use a Reaction to make one melee attack with a weapon or Unarmed Strike.
3. Resolve that attack immediately before the creature leaves reach; check Disengage and teleportation exceptions.

Example decision

A visible enemy walks out of a defender’s reach. The defender has an available Reaction and makes the Opportunity Attack. Where is the enemy for that attack?

Still within reach, just before leaving it.

The rule puts the attack immediately before the departure from reach. It is one melee attack paid for with the defender’s Reaction, not a complete Attack action. The stated scenario supplies visibility and ordinary movement; an effect such as Disengage or teleportation changes whether an Opportunity Attack is available.

Watch for this mistake

A trigger is not a free extra Reaction. If the Reaction has already been spent and has not refreshed, this general rule does not supply a replacement.

Ask the table

Can the defender see the departure, does it provoke, and is the Reaction available?

Source rule

You can make an Opportunity Attack when a creature that you can see leaves your reach using its action, its Bonus Action, its Reaction, or one of its speeds. To make the Opportunity Attack, take a Reaction to make one melee attack with a weapon or an Unarmed Strike against the provoking creature. The attack occurs right before the creature leaves your reach. See also “Playing the Game” (“Combat”).

Source: SRD 5.2.1, 185. Online guide.