When you make a melee attack, you can also mark the target. Until the end of your next turn, any [[Opportunity Attack]] you make against the marked target has [[Advantage]]. The [[Opportunity Attack]] doesn't expend your [[Reaction]], but you can't make the attack if anything, such as the [[Incapacitated]] condition is preventing you from taking [[Reaction|Reactions]]. You are limited to one [[Opportunity Attack]] against each Marked creature per turn.