Mike O'Brien is one of the three co-founders of ArenaNet and the leader of the Design and Content teams for Guild Wars. Previously, he worked as a company director and a lead programmer at Blizzard Entertainment where he developed the 3D rendering engine of Warcraft III: Reign of Chaos and led the development of Battle.net. He also worked on Warcraft II: Tides of Darkness, Diablo and StarCraft, where he, among other things, designed and created the MPQ archives used in all Blizzard games after Diablo. Mike O'Brien was featured as one of the most influential people in the computer and video game industry on PC Gamer's September 1999 cover story "Game Gods".

Mr. O'Brien also developed an old DOS ASCII game called "Pyro 2".