O
| obstetr/o | midwife |
| ocul/o | eye |
| odont/o | teeth |
| -oid | resembling |
| -ole | small, minute |
| olig/o | scanty |
| -oma | tumor |
| omphal/o | navel (umbilicus) |
| onc/o | tumor |
| onych/o | nail |
| oophor/o | ovary |
| -opaque | obscure |
| ophthalm/o | eye |
| -opia | vision |
| -opsia | vision |
| -opsy | view of |
| opt/o | eye, vision |
| optic/o | eye, vision |
| or/o | mouth |
| orch/o | testis (plural, testes) |
| orchi/o | testis (plural, testes) |
| orchid/o | testis (plural, testes) |
| -orexia | appetite |
| orth/o | straight |
| -ory | pertaining to |
| -ose | pertaining to; sugar |
| -osis | abnormal condition; increase (used primarily with blood cells) |
| -osmia | smell |
| oste/o | bone |
| ot/o | ear |
| -ous | pertaining to |
| ovari/o | ovary |
| ox/i | oxygen |
| ox/o | oxygen |
| -oxia | oxygen |
| oxy- | quick, sharp |
