Does the Booming Blade spell's damage from moving trigger before or after an opportunity attack?

Question

I hit an enemy with booming blade, and before my next turn they chose to move away, provoking an opportunity attack. Does the booming blade's damage-on-movement trigger before or after I make my opportunity attack?

Answer 1

The Booming Blade effect triggers before the opportunity attack.

Booming Blade states:

If the target willingly moves before then, it immediately takes 1d8 thunder damage, and the spell ends.

Opportunity Attack states:

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 interrupts the provoking creature’s movement, occurring right before the creature leaves your reach.

Emphasis mine

The creature first begins to move, triggering the Booming Blade and then, right before it leaves your reach, the opportunity attack triggers.

Of course this is all assuming you have not spent your reaction, your reach is 5ft and the enemy attempts to immediately leave your reach instead of moving around you.

Answer 2

Booming Blade triggers first.

Let's start with quotes from the specific abilities:

Booming Blade says:

If the target willingly moves before then, it immediately takes 1d8 thunder damage, and the spell ends.

Opportunity Attack says:

The attack interrupts the provoking creature’s movement, occurring right before the creature leaves your reach.

When not using a grid, a character stepping even one foot away has "willingly move[d]" and triggers Booming Blade. An opportunity attack follows if they continue to move and "leaves [the other character's] reach".

Fighting on a grid is an optional rule, and shouldn't have any material effect on how a spell plays out. It's more granular nature should not change the order things trigger. In effect, as soon as the victim declares their intention to move on a grid, Booming Blade triggers. The Opportunity Attack triggers when their movement is about to carry them to a square outside the character's reach, but before it actually happens (relevant for things like Sentinel).

Related "Trick"

Combine Booming Blade with War Caster. Hit the target with Booming Blade on your turn. If they move, they take the damage. When they provoke an opportunity attack, hit them with it again. If they keep moving, they take the damage again.