Odin was one of the main Asgard lords along with Thor, Freyr, and Aesir.
When the Goa'uld Forseti enslaved the Viking people of Earth, Odin cast him away from the Viking lands.(SG1: "The DVD Collection 10")
It is unknown how, but Odin became corrupted and started forbidden experiments on the planet Valhalla. (Stargate SG-1: Valhalla)