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?