He is often registered in many sources under the name Billy Neill or Neil because the Welsh Rugby Union didn't want him to sound like he was Irish.

O'Neill was first selected to play international rugby for Wales in a game against Scotland as part of the 1904 Home Nations Championship.

O'Neill played in all three matches of the 1905 Championship which saw Wales win the tournament and the Triple Crown for the fourth time.

He also missed the Wales game against the first touring South African team, though was fit and well to represent both Glamorgan and Cardiff against the Springboks.

O'Neill was back in the Welsh squad the next season playing two of the games in the 1907 Home Nations Championship, and was in all four matches of the 1908 tournament.