Why does the son have a different last name?
O’Brien and his wife divorced. When she remarried, the kids took the stepfather’s name.
From Peri's Wiki:
Gilpin was born in Waco, Texas, as Peri Kay Oldham, daughter of James Franklin Oldham, a broadcaster who became known as Jim O'Brien, and his wife Sandra Jo Hauck. After her parents divorced, her mother married Wes Gilpin in 1969. Gilpin then took her stepfather's surname.
So it appears that Marc's father's last name was Oldham. O'Brien was his father's stage name. And Gilpin was his stepfather's name.