| Obad. | (in the Bible) Obadiah |
| Obed. | obedience |
| obj. | object, objective |
| Obj. | object(s) |
| obl. | oblique |
| obs. | obsolete |
| Obs. | obsolete |
| Observ. | observation(s) |
| Obstet. | obstetric(s) |
| Obstetr. | obstetric(s) |
| Obstetr. Med. | obstetric medicine |
| occas. | occasionally |
| Occas. | occasional |
| Occup. | occupation(al) |
| Occurr. | occurrence |
| Oceanogr. | oceanography, oceanographic(al) |
| Oct. | October |
| OE | (in dates) Old English |
| O.E.D. | Oxford English Dictionary |
| Off. | official, office |
| Offic. | official |
| Okla. | Oklahoma |
| Ont. | Ontario |
| Ophthalm. | ophthalmology |
| Ophthalmol. | ophthalmology, ophtalmologic(al) |
| opp. | opposed (to), the opposite (of) |
| Oppress. | oppression |
| Opt. | optics |
| Orac. | oracle(s) |
| Ord. | order(s) |
| Ord. | ordinance(s) |
| Org. | organic |
| Organ. Chem. | organic chemistry |
| Org. Chem. | organic chemistry |
| orig. | origin(al)(ly) |
| Orig. | original |
| Orkn. | Orkney |
| Ornith. | ornithology |
| Ornithol. | ornithology, ornithologic(al) |
| Orthogr. | orthography |
| O.T. | Old Testament |
| Outl. | outline |
| Oxf. | Oxford |
| Oxfordsh. | Oxfordshire |
| Oxon. | Oxfordshire |
