| Word/Phrase | Definition | Useage |
| ö | island | Swedish |
| oäkta | illegitimate (child) | Swedish |
| Oakum Worker | Took old ropes apart for the hemp fibre (oakum ) to be used for caulking (qv) | Scot.Occup |
| oangiven | not given | Swedish |
| obefintliga | persons whose residence is not known | Swedish |
| obit sine prole (o.s.p.) | (he or she) died without offspring | Latin |
| obit | (he or she) died | Latin |
| Occupier | Tradesman | UK Occupation |
| och | and | Swedish |
| också | also | Swedish |
| octobre | October | French |
| octubre | October | Spanish |
| odaterad | not dated | Swedish |
| öde | vacant, fate | Swedish |
| odöpt | not christened | Swedish |
| Oedema | Fluid accumulation & swelling. See pulmonary edema | Scot.Dise 1855 |
| ofärdig | lame, crippled | Swedish |
| ofta | often | Swedish |
| öga | eye | Swedish |
| ogift | unmarried | Swedish |
| OGS | Ontario Genealogical Society | Colonial Diseases |
| Oil & Colour Man | Dealer in the victualling trade - not to be confused with the above! | UK Occupation |
| Oil Colour Man | Employed in the paint manufacturing trade, mixing pigmented paints | UK Occupation |
| Oil Miller | Miller of seed to extract oils for food manufacture. Rapeseed, cottonseed, linseed etc. Cottonseed oil was also used to lubricate the spinning mule. | UK Occupation |
| Oilman | Lamp oil seller | UK Occupation |
| okänd | unknown | Swedish |
| Oktober | October | Germany/Swedish |
| Olitor | Kitchen gardener – Olitory; a kitchen garden | UK Occupation |
| Oliver Smith/Man | A blacksmith who used a foot operated hammer called an Oliver | UK Occupation |
| olycka | accident, misfortune | Swedish |
| om | if, about | Swedish |
| omkommen | died accidentally, drowned | Swedish |
| omkring | about | Swedish |
| omvändelse | conversion | Swedish |
| omyndig | minor | Swedish |
| On the Club | This is not an occupation but is often seen in census returns. It refers the the person being out of work and drawing benefit from 'the Club', which was an appropriate insurance scheme. | UK Occupation |
| ondslag | syphilis | Swedish |
| Onkel | Uncle | Germany |
| onomastics | The science or study of the origin and forms of proper names of persons or places | Colonial Diseases |
| onsdag | Wednesday | Swedish |
| Onsetter | Performs the duties of a Banksman, except at the bottom of the shaft. | UK Occupation |
| öra | ear | Swedish |
| ordbok | dictionary | Swedish |
| Orderly | Military batman or officer's servant | UK Occupation |
| Ordinary Keeper | Innkeeper with fixed prices | UK Occupation |
| ormbett | bitten by a snake | Swedish |
| Orphan | Child whose parents are dead; sometimes, a child who has lost one parent by death. | General |
| Orphan's court | Orphans being recognized as wards of the states, provisions were made for them in special courts. | General |
| Orraman | Spare man or odd-job man around the farm | Scot.Occup |
| Orrery Maker | Made an apparatus which showed planetary movements - after the inventor, the Earl of Orrery | UK Occupation |
| Orrice Weaver | Designed / wove lace patterns in silk | UK Occupation |
| orsak | cause, reason | Swedish |
| ort | place | Swedish |
| ortförteckning | locality list, gazetteer | Swedish |
| ortodox | (Eastern) Orthodox | Swedish |
| Osier or Withy Peelers | Peeled bark from willow stems (osiers) for basket weaving - usually women & children | UK Occupation |
| Osler | Bird catcher | UK Occupation |
| öster (östra) | east (eastern) | Swedish |
| Ostiary | Monastery doorkeeper | UK Occupation |
| Ostler / Hostler | Originally, a Hostler was the host of an Inn or (H)ostelry. Later became the man employed to look after the horses of the visitors | UK Occupation |
| Ostler | Hostler, person looking after horses at an inn | Scot.Occup |
| otidigt sängelag | fornication betweeen a couple who intends to marry (who are not engaged) | Swedish |
| Otitis media | Infection of the middle ear | Scot.Dise 1855 |
| ottobre | October | Italian |
| Ourman | Overseer | Scot.Occup |
| Out Crier | Auctioneer | UK Occupation |
| Outworker | Home-worker, particularly in weaving | UK Occupation |
| Oval Turner | Lathe operator who produced oval shaped wooden legs of tables, chairs, stair balusters. etc. | Scot.Occup |
| ovan, ovanför | above | Swedish |
| över | over | Swedish |
| överlevande | survivor(s) | Swedish |
| Overlooker | Overseer or foreman esp. in textile mills | UK Occupation |
| Overman | Colliery supervisor - checked the workers and coal quality | UK Occupation |
| Overmantel Maker | Craftsman who made ornamental structures over a mantelpiece. | UK Occupation |
| Overseer | The officials who performed the assessment and collection of the poor-rates. More Info. | UK Occupation |
| övre | upper | Swedish |
| Owler | Smuggler - particularly sheep or wool | UK Occupation |
| oxel | tumour | Swedish |
| Oyster Dredger | Crew member on an oyster boat | UK Occupation |
| Oyster Hawker | Street seller of pickled oysters - 18/19th century | UK Occupation |