Complex sentence: The son of either Gunnar or Oslac, northern ealdormen, if the former he may had attained adulthood by the 960s, when a man of his name raided Westmorland.
Split into simpler sentences: The son of either Gunnar or Oslac, a northern ealdormen. If he was the former, he may had attained adulthood by the 960s, when a man of his name raided Westmorland.