O
- oak trees (2)
- obelisks (1)
- observatories (4)
- octagonal buildings (8)
- octagonal towers (1)
- office buildings (226)
- officers' clubs (4)
- officers' quarters (219)
- offices (382)
- official residences (8)
- oil houses (5)
- oil storage tanks (5)
- oil wells (9)
- one-room schools (4)
- onion domes (1)
- open plan buildings (1)
- open-air theaters (2)
- open-hearth furnaces (3)
- opera houses (6)
- operas & operettas (1)
- orangeries (2)
- ordinaries (taverns) (1)
- ordnance industry (12)
- ordnance testing (1)
- ore docks (3)
- ore industry (10)
- organizations' facilities (2)
- organizations (124)
- orphanages (4)
- orphans (2)
- Orthodox churches (8)
- outbuildings (91)
- outfall sewers (4)
- overpasses (10)