You can prepare an attack if a condition is met to use your Opportunity attack against a target which might not meet the initial conditions of and Opportunity attack.

Examples:

1. When an enemy moves from cover with a ranged attack weapon, you attack with your bow.
2. When an enemy comes within reach of your melee weapon.

Keep in mind the initiator, or target, might be able to get their attack off first via their own special attack ability, like Charge!! before your conditional Opportunity attack is able to go off.

Using this still means your reaction is still used for your Opportunity attack, because it might trigger outside of your turn as well.