Jarl Olaf is initially a vassal of the Kingdom of Nords. You may fight him or have him side with you as you help Lethwin Far-Seeker, the corresponding claimant, overthrow King Ragnar and gain control of the kingdom. In Warband, Jarl Olaf's banner is a Rhodok-like crossbow on top of a half blue, half green background. This is rather odd, since Nord troops don't make use of crossbows.