oari- or oario- See: ovari(o)-.

ob- [Latin ob toward, against, in the way of] Usually denotes opposition (obstruct).

oct- or octa- or octav- or octavo- or octo- [from Latin octo, octavus eight] Eight (octoploid, octipara).

ocul- or oculo- [from Latin oculus eye] Denotes the eye (oculomycosis, oculomotorius, oculonasal, oculozygomatic).

odont- or odonto- [from Greek odous, odont- tooth] Denotes a tooth or teeth (odontogenesis, odontology, odontopathy).

odyn- or odyno- [from Greek odyne pain] Denotes pain (odynacusis, odynometer).

oec- or oeco- See: ec(o)-

oen- or oeno- (also en- or eno-) [from Greek oinos wine] Denotes wine or something wine-colored (enology, Oenochrominae, Oenomys)

oesophag- or oesophago- See: esophag(o)-

oiko- See: ec(o)-

olea- [from Latin olea oil, olive, olive tree] oil (oleaginous).

olfact- [from Latin olfacere to smell] Denotes smell (olfactive, olfactology, olfactory).

olig- or oligo- [from Greek oligos few, little, scanty, small] A few, a little, small (Oligocene, oligosaccharide).

om- or omo- [from Greek omos ulna, omia shoulder] Denotes the shoulder (omitis, omodynia).

ombro- [from Greek ombros rain] Rain (ombrology, ombrophile, ombrophilous).

oment- or omento- [from omentum] Omentum (omentotomy).

omni- [from omnis every, all] All (omnivorous).

omphal- or omphalo- [from Greek omphalos navel, umbilical cord] (1) navel (omphalitis); (2) the umbilicus, or umbilical cord (omphalos).

onco- [from Greek onkos bulk, mass] Tumor, cancer, tumor causing (oncogene, oncology).

onc- or onco- or onch- or oncho- or onci- [from Greek onkinos hook] Hook, hooklike, barbed (onchium, oncosphere). Compare: unci-.

ont- or onto- [from Greek on being (pres. participle of einein)] Being, living thing (ontogeny).

onych- or onycho- or onyx- [from Greek onyx talon, claw] Condition of the fingernails or toenails of the type indicated by the stem or suffix (onycholysis, onychoma, onychosis, onyxis).

oo- [from Greek oion egg] Denotes an egg or the primodial cell that develops into an ovule (oocyte, oogamous, oogenesis).

oophor- or oophoro- [oo- + -phor] Denotes an ovary (oophorocystosis, oophorectomy).

ophthalm- or ophthalmo- [from Greek ophthalmos eye] Eye, eyeball, of or affecting the eyes (opthalmoscope).

opto- or optic- or optico- [from Greek optikos of or for sight] Denotes vision or eye (optostriate).

or- or oro- [from Latin os, oris mouth] Mouth (orad, oral, oropharynx).

orchid- or orchido- or orcheo- or orchio- [from Greek orchis testicle] Testicle (orchidectomy, orchiodynia).

ore- or oreo- or oro- [from Greek oros mountain] Mountain (orogeny).

orexi- [from Greek orexis desire, appetite] Appetite (orexigenic).

ornith- or ornitho- [from Greek ornis bird] Bird (ornithophily, ornithology).

ortho- [from Greek orthos straight, correct, true, just] (1) straight (Orthoptera); (2) normal, average (orthopedic); (3) at a right angle (orthopnea); (4) chemistry: in ortho-substitution, the substituents occupy positions 1 and 2 (see explanatory diagram>>).

os- [from Latin os, oris mouth] Mouth (oscedo).

osch- or oscheo- [from Greek osche scrotum] Scrotum (oschitis, oscheohydrocele).

osmo- [from Greek osme smell, odor, sense of smell] Odor, smell (osmology).

osmo- [from Greek osmos thrusting] Pertaining to osmosis (osmotic).

osphresio- [from Greek osphresis smell] Denotes odor or smell (osphresiology).

ossi- [from Latin os, ossis bone] Bone (ossicle).

ost- or oste- or osteo- [from Greek osteon bone] Bone (ostemia, osteectopia, osteofibroma).

ostra- or ostre- or ostrei- or ostreo- [from Latin ostrea oyster] Oyster (Ostreidae, ostreophagous).

ostrac- or ostraco- [from Greek ostrakon shell] Shell (Ostraciidae).

ot- or oto- [from Greek ous, ot- ear] Denotes the ear or hearing (otalgia, otic, otitis, otology).

ov- or ovi- or ovo- [from Latin ovum, ovi egg] Denotes egg or ovum (ovary, oviduct, oviparous, ovum, ovipositor).

ovari- or ovario- (also oari- or oario) [from Latin ovarium, ovarii ovary] Ovary (ovariectomy, ovariotubal).

ovul- or ovulo- [ov- + -ul] Denotes ovule or ovum (ovulation).

ox- or oxo- or oxy- [from Greek oxy- sharp-] (1) sharp in shape, pointed (oxycephalic); (2) sharp in sensation (oxypathia); (3) containing oxygen (oxyhemoglobin).