Jon Øyvind Andersen, mostly known as Jardar, is a Norwegian black metal guitarist. He was one of the founding members of the band Old Man's Child, with Galder and Tjodalv. He has played on all the bands albums, except Ill-Natured Spiritual Invasion (1998) , Vermin (2005) and Slaves of the World (2009) , where Galder was the sole guitarist. Besides as a guitarist, Jardar has also contributed with backing vocals, and he was also co-writer on two songs on In Defiance of Existence.
In 2005 he formed the band Insidious Disease, together with Silenoz, the Dimmu Borgir guitarist, the bass player Shane Embury from Napalm Death and drummer Tony Laureano.