Complex sentence: If derived from the Irish Gaelic, "Ó hÓgáin", it is diminutive of "Og" meaning "young", but if it is derived from Cornish, it means mortal.
Split into simpler sentences: If derived from the Irish Gaelic, "Ó hÓgáin", it is diminutive of "Og" meaning "young". If it is derived from Cornish, it means mortal.