null - oner
- oil well
- oil well drilling
- oir
- OK bill of lading
- old age
- Old Bailey
- old line company
- old line policy
- Old Natura Brevium
- old school
- old school physician
- old style calendar
- Old-Age and Survivors' Insurance
- Old-Age and Survivors' Insurance Act
- old-age assistance
- old-age exemption
- oleo oil
- oleomargarine
- Oleron
- oligarchy
- olograph
- olographic will
- om
- ome bueno
- omissa vel male appretiate
- omissio
- Omissio eorum quae tacite insunt nihil operatur
- omission
- omissis omnibus aliis negotiis
- omissus
- omits to provide
- omitted property
- omme
- Omne actum ab intentione agentis est judicandum
- Omne crimen ebrietas, et incendit et detegit
- Omne jus aut consensus fecit, aut necessitas constituit, aut firmavit consuetudo
- Omne magis dignum trahit ad se minus dignum sit antiquius
- Omne magis dignum trahit ad se minus dignum, quamvis minus dignum sit antiquius
- Omne magnum exemplum habet aliquid ex iniquo, quod publica utilitate compensatur
- Omne majus continet in se minus