This person plays Old School Runescape, and generally uses shortened runescape slang such as "Oic" (oh i see), "pot up" (ready your potions), or "red barred" (someone/thing that has very low hitpoints). They appear to be a pay-to-play (membership) user, as they have decent gear not seen by free-to-play players. They seem to know most the ins and outs of the game as well, being fairly skilled.