null - oner
- ocean going steamer
- ochlocracy
- octabis
- octave
- octo tales
- octoroon
- ocular refraction
- oculist
- odd lot
- Oderunt peccare boni, virtutis amore; oderunt peccare mali, formidine poenae
- odhal or odhall
- odio et atia
- Odiosa et inhonesta non sunt in lege praesumenda
- Odiosa non praesumuntur
- odious plea
- odium
- odium spoliatoris
- odometer
- odor
- oeconomicus
- oeconomus
- oecumenical
- oeps
- oes
- of
- of a civil nature
- of age
- of counsel
- of course
- of new
- of right
- Of two possible constructions, adopt the one which will save and not destroy
- off-the-road production
- offa execrata
- offend
- offense
- offense against the United States
- offense malum in se
- offense malum prohibitum
- offensive