Øystein Greni is the son of Thor Sigbjørn Greni, singer and songwriter of the Undertakers Circus, a 60/70s soul band. In 1992, Greni, Erik Tresselt and Christer Engen formed Bigbang which has since established its position as one of the principal rock acts of Norway. As the only consistent member throughout the band's history, Bigbang's success is often attributed to Greni's skills as a guitarist and songwriter.
Greni also works as a music producer. In 2003, he produced the EP debut of Latin performer Maria Orieta (Greni's fiancée) Buenos Aires via the Grandsport Records indie label.
Greni is also a former skateboarding champion, having won the 1991 European Skateboard amateur championship in Antwerp. In 2007 Greni and the band relocated to U.S.A.