In what order do simultaneously-triggered opportunity attacks happen?

Question

My GM has decided that opportunity attacks happen in an arbitrary order with respect to persons making the attack, instead of all at the same time — and the order isn't even in the order of initiative. I was under the impression that all OAs on a target happened simultaneously. Is this not the case?

The specific example from our game is that in response to my movement, a monk punched me and then a cleric cast Command (via the War Caster feat) on me, both using their OAs.

I argued that the Command spell shouldn't work in that circumstance for a bunch of reasons:

1. The spell says that its effect happens on your next turn, not the same turn.

2. The command was to "Grovel" and it put my character in danger, namely from the Monk's punch and the fact that my PC was surrounded by enemies.

Even assuming that each of those opportunity attacks was valid, is it correct that the opportunity attackers get to decide the order among them?

Answer 1

As it's your turn, you get to decide!

Xanathar's Guide to Everything (p. 77) contains an optional rule to help adjudicate simultaneous effects:

If two or more things happen at the same time on a character or monster's turn, the person at the game table-whether player or DM-who controls that creature decides the order in which those things happen. For example, if two effects occur at the end of a player character's turn, the player decides which of the two effects happens first.

So if multiple events would occur simultaneously (such as multiple opportunity attacks reacting to the same trigger), then the player whose turn it currently is gets to decide in what order those events occur for mechanical purposes.

Answer 2

The rules for opportunity attacks state:

Opportunity attacks (PHB p.195)

To make the opportunity attack, you use your reaction to make one melee attack against the provoking creature. The attack interrupts the provoking creature’s movement, occurring right before the creature leaves your reach.

Assuming a number of creatures all get an opportunity attack on another creature they all interrupt the creature's movement, which occurs on a specific initiative order. It can then be said that all of the reactions tie on that initiative order and this is handled in the initiative rules:

Initiative (PHB p.189)

If a tie occurs, the DM decides the order among tied DM-controlled creatures, and the players decide the order among their tied characters. The DM can decide the order if the tie is between a monster and a player character. Optionally, the DM can have the tied characters and monsters each roll a d20 to determine the order, highest roll going first.

So in the case of tied reactions, if the creatures with the opportunity attacks are "monsters" the DM decides. If they are all player characters then the players decide. If it is a mix of "monsters" and PCs then the DM decides.

Optionally all the tied combatants can roll d20s (unmodified, the rules say nothing about a modifier) and go in that order. There is no guidance on what happens with another tie in this optional rule, but I think common sense would be to roll again between those who are tied until it is resolved.

Answer 3

First, you have used "AoOs" when you mean "Reactions". The rules have this to say about Reactions:

Certain special abilities, spells, and situations allow you to take a special action called a reaction. A reaction is an instant response to a trigger of some kind, which can occur on your turn or on someone else's. The opportunity attack, described later in this section, is the most common type of reaction.

When you take a reaction, you can't take another one until the start of your next turn. If the reaction interrupts another creature's turn, that creature can continue its turn right after the reaction.

The rules are completely silent on what happens if two creatures have their reactions triggered by the same event. Therefore, it is up to your DM to decide if they happen simultaneously and, if not, the order in which they happen.

It appears that your DM has decided that the creatures who have their reactions triggered can decide between them who goes first. I find this decision ... problematic, but it was not my decision to make. However, now that you know this is the way reactions work, I am sure you and your fellow players can turn this to your advantage in future combats.