Does moving away from a prone creature trigger an opportunity attack?

Question

I will give you the situation this came up in, but essentially I would like to know whether moving away from a prone creature would trigger an opportunity attack or not.

If you move 20 feet to attack as a Warhorse while in druid Wild Shape, the creature needs to do a DC 14 Strength check or be knocked prone. (PHB p.311)

If the target fails and is knocked prone you can make a bounus attack with your hooves.

If you want to move away to be able to charge again on your next turn, would this normally trigger an opportunity attack?

I am not sure because the creature would have the prone condition, so guidance with this would be helpful.

Answer 1

Yes, the OA triggers, but they attack at disadvantage.

The moving away still triggers an opportunity attack ...

Opportunity Attacks

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 creature. The attack occurs right before the creature leaves your reach.
(Basic Rules, p. 76)

... but the Prone Condition in Appendix A shows that the OA(which requires an attack roll) has disadvantage.

Prone • The creature has disadvantage on attack rolls.

Experience: I have used this situation to our party's advantage with some frequency in the past few years, as did my playing companions. I had the Shield Master feat; when the ruling changed a few years ago my DM no longer allowed me to first knock enemies down and then attack them with my sword. But I kept knocking enemies down ... because I could. We found out that me knocking the enemy down often allowed one of our other PCs to move in and attack, and risk the OA. With disadvantage on the OA it was usually a worthwhile risk.

Answer 2

Yes, prone creatures can make opportunity attacks.

The base understanding in D&D 5E is "Rules do what they say they do." So...

Rules for opportunity attacks

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 creature. The attack occurs right before the creature leaves your reach.

You can avoid provoking an opportunity attack by taking the Disengage action. 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.

So, in short, to make an opportunity attack...

• The attacker must have a Reaction available
• A Creature must move out of the attacker's reach using its movement, an action, or a reaction

Prone does the following:

-A prone creature's only movement option is to crawl, unless it stands up and thereby ends the condition.
-The creature has disadvantage on attack rolls.
-An attack roll against the creature has advantage if the attacker is within 5 feet of the creature. Otherwise, the attack roll has disadvantage.

Nothing in that list prevents using a Reaction. The Opportunity Attack will be made at Disadvantage, though.