There are at least two acts with this name:
1) OK Band (also know as Goran & OK Band, Goran & OK Bend or OK Bend) is a band from Serbia. They play pop-rock music. The name of frontman is Goran.
2) O.K. Band is a band from Osijek, Croatia.
3) Czech synth-pop group. Site: okband.estranky.cz