Do creatures with a reach of 0 get attacks of opportunity when a foe moves away?

Question

Some creatures like swarms have a reach of 0 with their attacks-- do they even get attacks of opportunity when someone leaves their reach?

Answer 1

Zero foot reach means attacking only in the same combat space.

tl;dr 0-reach creatures do get an opportunity attack.

Swarm example

Things like a swarm of insects can occupy the same space as an opponent.

Swarm. The swarm can occupy another creature's space and vice versa

In the case of an opponent moving out of the same space as the swarm, the swarm would get an attack of opportunity. They do not get attacks of opportunity against adjacent spaces as they can't reach.

Melee Attacking with 0' reach means the same space.

Reach dictates how far away a creature can melee attack an opponent. This is also the distance at which they make attacks of opportunity.

Most creatures have a 5-foot reach and can thus attack targets within 5 feet of them when making a melee attack.

In combat, an adjacent space is 5 feet away when using a 5 foot grid. Similarly, a combat space is zero feet away from itself, so a creature with a 0 foot reach could only attack (and opportunity attack) a creature that is in the same space.

Opportunity Attack Timing and Reach

The rules of opportunity attack are copacetic with a creature that has 0 reach as the attack happens right before the opponent moves out of the space (leaving reach):

You can make an opportunity attack when a hostile creature that you can see moves out of your reach.... The attack occurs right before the creature leaves your reach.

This also means that an opponent moving through the spaces adjacent to a creature with 0 reach does not incur an opportunity attack as the opponent is not entering then leaving the reach of the creature.