The midfielder nodded home a Lee Brown free-kick from the corner of the box to deny Oxford a win they looked like achieving for most of the game.
Rovers thought they had equalised after 31 minutes when Kaid Mohamed's header was deflected in by O'Toole, who was judged offside.
Oxford should have wrapped up the points just before the hour mark when the dangerous Constable teed up Josh Ruffels, who blazed over from 12 yards.
Tom Locker had chance to level for Rovers six minutes later but his low shot was well saved by Ryan Clarke.
Both sides had to contend with a gusty wind and tricky playing surface, which made it difficult to control the ball.
- Sports & Recreation
- James Constable
- Kaid Mohamed
- Dave Kitson