Cómo han dicho el ataque de oportunidad es un ataque de melee, pero deberías repasar si existe alguna dote que te dé la oportunidad de efectuarlo de otras formas, yo con mi Ranger lo hago con la dote
"Snap shoot"
With a ranged weapon, you can take advantage of any opening in your opponent’s defenses.
Prerequisite: Dex 13, Point-Blank Shot, Rapid Shot, Weapon Focus, base attack bonus +6.
Benefit: While wielding a ranged weapon with which you have Weapon Focus, you threaten squares within 5 feet of you. You can make attacks of opportunity with that ranged weapon. You do not provoke attacks of opportunity when making a ranged attack as an attack of opportunity.
Normal: While wielding a ranged weapon, you threaten no squares and can make no attacks of opportunity with that weapon.
Espero te sea útil, un saludo