Osbeorn, also spelled Osbjorn and Osbert, given the nickname Bulax, was the son of Siward, Earl of Northumbria. He is one of two known sons of Siward, believed to be the elder. While it is normally assumed he was the son of Siward's Bamburgh wife Ælfflæd, it has been suggested by William Kapelle that Osbeorn's mother was not Ælfflæd. The nickname "Bulax" probably represents the Old Norse term for "Poleaxe".