| Observer - Someone selected to aid the referee in a competition to ensure that it is played in agreement with the Rules of Golf. |
| Obstruction - Any man-made object that obstructs play. |
| One up - To score one hole more than your opponent in match play. |
| One-putt - When a golfer only takes one putt on the green to hole the ball. |
| One-wood - Another name for a driver. |
| Open - This is a competition where both professionals and amateurs can play. |
| Open stance - When a golfer addresses the ball with their front foot closer to the ball line than their back foot. |
| Out - The first nine holes of an 18 hole golf course. |
| Out of bounds - The area neighbouring the golf course, where play is forbidden. |
| Outside agency - Someone who is not a golfer in a competition i.e. referee, spectator. |
| Overclub - When a golfer uses a club that gives too much distance. |
| Overlapping grip - When a golfer grips the club so that their little finger of one had is wrapped around the fore-finger of the other. |