| Picture File | Related File | Code |
|---|
| Oases | | OAS |
| Oaths | | OAT |
| Obelisks | | OBEL |
| Obesity | | OBES |
| Observatories | | OBS |
| Occult, The | | OCC |
| | Alchemy | ALC |
| | Alphabets–Hebrew | ALPH–(A–L) |
| Occupational therapy | | OCCU |
| Oceania | | OCE |
| Oceania — Life | | OCE–Lif |
| Oceanography | | OCEA |
| Octopuses | | OCT |
| | Cuttlefish | CUTT |
| | Squids | SQUI |
| Office buildings | | OFF–Bui |
| Office buildings — Interiors | | OFF–Bui–Int |
| Office equipment | | OFF–Equ |
| Office furniture | | OFF–Fur |
| Office workers | | OFF–Wor |
| Offices — 1899 & earlier | | OFFI–18 |
| Offices — 1901s –1920s | | OFFI–190 |
| Offices — 1930s | | OFFI–193 |
| Offices — 1940s –1950s | | OFFI–194 |
| Offices — 1960s | | OFFI–196 |
| Offices — 1970s | | OFFI–197 |
| Offices — 1980s | | OFFI–198 |
| Offices — 1990s | | OFFI–199 |
| Offices — 2001s | | OFFI–200 |
| Ohio | | OHI |
| Ohio — 1899 & earlier | | OHI–18 |
| Ohio — A–Z | | OHI–(A–Z) |
| Ohio — Cincinnati | | OHI–Cin |
| Ohio — Cincinnati–1899 & earlier | OHI–Cin–18 |
| Ohio — Cleveland | | OHI–Cle |
| Ohio — Life | | OHI–Lif |
| Oils & fats | | OIL |
| | Olive oil | OLI |
| Oklahoma | | OKL |
| Oklahoma — 1899 & earlier | | OKL–18 |
| Oklahoma — A–Z | | OKL–(A–Z) |
| Oklahoma — Industries | | OKL–Ind |
| Oklahoma — Life | | OKL–Lif |
| Olive oil | | OLI |
| Olympic games | | OLY |
| Olympic games — 1901s–1920s | | OLY–190 |
| Olympic games — 1930s | | OLY–193 |
| Olympic games — 1940s | | OLY–194 |
| Olympic games — 1950s | | OLY–195 |
| Olympic games — 1960s | | OLY–196 |
| Olympic games — 1970s | | OLY–197 |
| Olympic games — 1980s | | OLY–198 |
| Olympic games — 1990s | | OLY–199 |
| Olympic games — 2001s | | OLY–200 |
| Olympic games — Ancient | | OLY–Anc |
| Oman | | OMA |
| Opening ceremonies | | OPEN |
| Opera houses — A–L (Subdivided by country) | | OPER–(A–L) |
| Opera houses — England | | OPER–Eng |
| | London–Covent Garden | LON–Cov |
| Opera houses — France | | OPER–Fra |
| | Paris–Opera house | PARI–Ope |
| Opera houses — M–Z (Subdivided by country) | | OPER–(M–Z) |
| Opera houses — United States | | OPER–US |
| | New York City–Lincoln Center | NEW YC–Lin |
| | New York City–Opera houses | NEW YC–Ope |
| Operas (Subdivided by composer) — A–B | | OPERA–(A–B) |
| Operas — C–D | | OPERA–(C–D) |
| Operas — Cartoons | | OPERA–Car |
| Operas — E–G | | OPERA–(E–G) |
| Operas — H | | OPERA–(H) |
| Operas — I–K | | OPERA–(I–K) |
| Operas — L | | OPERA–(L) |
| Operas — M | | OPERA–(M) |
| Operas — Mozart | | OPERA–Moz |
| Operas — N–O | | OPERA–(N–O) |
| Operas — P–Q | | OPERA–(P–Q) |
| Operas — R | | OPERA–(R) |
| Operas — S | | OPERA–(S)_ |
| Operas — Sullivan | | OPERA–Sul |
| Operas — T–V | | OPERA–(T–V) |
| Operas — Verdi | | OPERA–Ver |
| Operas — W–Z | | OPERA–(W–Z) |
| Operas — Wagner | | OPERA–wag |
| Opium | | OPI |
| Optical illusions | | OPTI |
| Optometry | | OPTO |
| Oracles | | ORA |
| Orders | | ORD |
| Orders — British | | ORD–Bri |
| Orders — Ceremonies | | ORD–Cer |
| Orders — Russian | | ORD–Rus |
| Oregon | | ORE |
| Oregon — 1899 & earlier | | ORE–18 |
| Oregon — A–Z | | ORE–(A–Z) |
| Oregon — Coast | | ORE–Coa |
| Oregon — Industries | | ORE–Ind |
| Oregon — Lakes | | ORE–Lak |
| Oregon — Life | | ORE–Lif |
| Oregon — Mountains | | ORE–Mou |
| Oregon — Rivers | | ORE–Riv |
| Organized Crime | | ORG |
| Orgies | | ORGI |
| Orphans | | ORP |
| | Child welfare | CHIL–wel |
| | Homes (Institutional) | HOME |
| Outriggers | | OUT |
| Oxygen | | OXY |
| Oysters | | OYS |
| | Pearl fishing & industry | PEAR |
| | Shells (Mollusk) | SHE |
