Nuntio, I announce; nuncio, annunciation, pronounce.
Nutrio, I nourish; nutriment, nurse.
Octo, eight; octave, octagon, October.
Oculus, the eye; oculist.
Odor, smell; odour, redolent.
Omnis, all; omnipotent, omniscient.
Onus (onĕr-is), a burden; onerous.
Opus (opĕr-is), a work; operate, operation.
Ordo (ordĭn-is), order; ordinal, ordinary.
Oro, I pray; oration, inexorable, peroration.