O
- out of sight
- out of place
- out of breath
- out of order
- once in a while
- out of doors
- out of date
- out of control
- once for all
- once again
- on time
- on the whole
- on the spot
- on the road
- on the point of
- on the other hand
- on the ground of
- on the contrary
- on second thoughts
- on schedule
- on sale
- on purpose
- on occasion
- on hand
- on foot
- on fire
- on guard
- on earth
- on duty
- on condition that