omnis, all: omnibus, omni-scient, omnipotent.
ŏnus (ŏnĕr-), burden: onerous, ex-onerate.
ops, means; ōptimus, best: opulent, optimist, optimates.
ŏpus (ŏpĕr-), work; ŏpĕra, service: operate, operative, co-operation, opera.
opt-o (optāt-), I desire: option, adopt, adoptive, optative.
ordo (ordĭn-), order, row; ordĭn-o (ordĭnāt-), I set in order; orno (ornāt-), I deck: ordinal, extra-ordinary, ordination, co-ordinate, in-subordination, orna-ment, ad-orn, ornate; through Fr., order, ordain.
oriens, rising (sōl oriens, the east); orīgo (orīgĭn-), source: orient, orientalist, orientation, origin (Fr.), originality.
ōs (ōr-), mouth; ōr-o (ōrāt-), I speak, pray; ōmen (omin-), token: oral, or-acle, o-rotund, ori-fice (facio), os-cul-ation, adore, oration = orison.
ōvum, egg; ōvātio, an ovation: oval, ovary, ovi-form, ovi-parous (pario).
Paenīt-ēre, to be sorry: through Fr., penitent, repentance, penance.