all - and
- an open field
- an open hand
- an open heart
- an open letter
- an open mind
- an open question
- an open secret
- an opening gambit
- an optical illusion
- an ounce of discretion is worth a pound of wit
- an outside job
- an overflow meeting
- an owl car
- an oyster part
- an ugly duckling
- an unappropriated blessing
- an unearthly hour
- an unfortunate man would be drowned in a teacup
- an unknown quantity
- an unlicked cub
