If you move into melee range and then back out on the same turn, does it provoke an opportunity attack?

Question

I feel like this question has been asked before, but I couldn't find it.

I know that if you are engaged in combat with someone (or at least in melee range of someone's weapon, 5 feet normally or 10 with polearms) and you move away willingly on your turn, your opponent can make an opportunity attack. I am also aware that if you are engaged in melee combat with a target, you can sidestep around them without them being able to make an opportunity attack if you stay within 5 feet (or 10 for polearms).

What I want to know is, on your turn if you start OUT of the melee range, then enter the melee range (whether by passing or attacking), and then leave the melee range, can your opponent then make an opportunity attack? More specifically for movement purposes.

So if a monk runs past a bandit when he previously wasn't in Melee range before, can the bandit make the attack? Or does he have to start his turn in Melee range and then move?

Answer 1

Yes

A creature does provoke an opportunity attack when it moves into melee range and then back out on the same turn.

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 described scenario is exactly the reason why OAs were introduced — without them, hit-and-run tactics were superior for melee combatants.

There are few ways not to provoke an OA when moving out of somebody's reach:

• Being unseen
• Unwilling movement
• Teleporting
• Using disengage action
• Special feats and features, etc.

See Player's Handbook, page 195:

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.

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As @V2Blast pointed out in a comment, keep in mind that you have to expend your Reaction in order to make an opportunity attack, and you have only one Reaction until the start of your next turn. Basically you can make no more than one OA per round. The same applies for your enemies, unless they have a specific feature.

In addition, @PJRZ pointed out that there are features that change the action economy for the Disengage action. For example, the Goblin's Nimble Escape trait (which both the monster and the playable race have) allows it to use a Bonus Action to Disengage rather than an Action. (Rogues' 2nd-level Cunning Action feature allows them to do the same.) In addition, the Owl's Flyby feature means that it does not provoke opportunity attacks at all when flying out of an enemy's reach.