Allan Conn, I think you are agreeing with me - it is just a question of spelling.
We are not talking about something called an "o", which happens to belong to a king, queen, prince or princess. We are talking about something known as "a royal", with the "Oh" or "O" being there to fill up the line of verse, as in "Green Grow the Rushes":

I'll sing you one, oh
Green grow the rushes, oh
What is your one, oh?