One of Arthur’s warriors in Welsh legend. He carried a knife called Bronllafyn (Bronllavyn) Short Broad, which was so big that it could be used as a bridge for armies to cross bodies of water. Osla accompanied Arthur on the epic hunt for Twrch Trwyth. At the Severn river, Osla lost his knife from the sheath while running after the boar; the sheath filled with water and pulled Osla to the bottom of the river.

He features in the Mabinogion story of the The Dream of Rhonabwy as an adversary of Arthur at Badon. He is perhaps, then, to be identified with Octa, the son or grandson of the Saxon Hengist.