Can you use an Opportunity Attack on a Marked enemy if you already used your Reaction?

Question

The DMG introduces an optional rule called "Mark" (DMG, p. 271, bold added)

When a creature makes a melee attack, it can also mark its target. Until the end of the attacker's next turn, any opportunity attack it makes against the marked target has advantage. The opportunity attack doesn't expend the attacker's reaction, but the attacker can't make the attack if anything, such as the incapacitated condition or the shocking grasp spell, is preventing it from taking reactions. The attacker is limited to one opportunity attack per turn.

Does having already used your reaction this round (for example, having casted the Shield spell during the previous turn) count as something that "is preventing it from taking reactions"? Or could you still take an opportunity attack against a marked target, even if you'd already used your Reaction earlier in the round?

Answer 1

Yes.

You may make this special opportunity attack against a marked opponent that does not cost your reaction even if you have spent your reaction.

Having already spent your reaction does not "prevent you from taking reactions." It just means you have nothing to spend. This wording is meant to restrict this free opportunity attack to those times when you could have used a normal opportunity attack while decoupling it with the cost of spending your reaction.

An analogy would be going to a store with not enough money. Nothing is preventing you from from going to the store. You just don't have enough money to spend when you get there.

This is in contrast to your car having a flat tire. It doesn't matter how much money you could spend, you can't even get to the store (without outside help).

For me this is a matter of how many resources (1 reaction at most) you have and what the costs are (no reaction) as opposed to conditions or situations which completely prevent you form attempting an opportunity attack in the first place.

Answer 2

We have a similar wording to "doesn't expend the attacker's reaction" in the Ritual spells.

The ritual version of a spell takes 10 minutes longer to cast than normal. It also doesn’t expend a spell slot.

It is probably intended that, even if there are no spell slots remaining, the Ritual can still be cast.

Similarly, anything that prevents you from casting spells will prevent you from casting Ritual spells, as the incapacitated condition mentioned in the Mark. Arguably, "having no spell slots left" "prevents" you from casting spells that are not cantrips.

From analogy and very similar wording, I would say that no, simply having no reaction left is not something that "prevents you from taking a reaction". (Or, answering the title: Yes, you can still make the attack.)

In particular, if that was intended, it would probably be explicitly stated, at least in the examples.