Attack of Opportunity vs Ready Action order

[Rain Maker]

Hi guys! My barbarian ready a action to attack a monster if it's leaves a protection position. In touch with monster, has been a warrior and the monster action triggered his oportunity attack. What occurs first, my ready action or opportunity attack?

Other point: warrior is a master's character. Has any priority to player's characters (considering first rule of game)?

[Nicodranas]

If I was the GM I would follow the initiative order to determine which reaction goes first