O

The surname prefix "O" is of Irish and Scottish Gaelic origin, meaning "grandson" or, more broadly, "descendant." It is equivalent to the Welsh "Ap" and the Anglo-Norman "Fitz."1

In Ireland, "O" names became common from the 11th century onwards, often formed from a male ancestor's Christian name. For example, "O'Neill" signifies a descendant of Neill. While "Mac" names are more prevalent in Ulster, "O" names are more common in other parts of Ireland.1

It's a common misconception that the "Mc" form of a prefix is exclusively Scottish and "Mac" is Irish; both are used interchangeably, with "Mc" being an abbreviation of "Mac."1