Disclosed is a method for providing an online game, comprising the steps of: maintaining an event database recording an aggressive point generation function corresponding to each predetermined event; receiving a particular event from a player character; identifying an aggressive point generation function corresponding to the particular event by referring to the event database; generating aggressive point information according to receipt of the particular event, on the basis of the identified aggressive point generation function; recording the generated aggressive point information in a predetermined monster information database in association with the player character; and determining a target for attack of a monster on the basis of the aggressive point information recorded in the monster information database, and a system thereof.