O
- oak trees (1)
- obelisks (1)
- observatories (4)
- octagonal buildings (8)
- octagonal towers (1)
- office buildings (212)
- officers' clubs (4)
- officers' quarters (203)
- offices (323)
- official residences (9)
- oil houses (5)
- oil storage tanks (3)
- oil wells (9)
- one-room schools (2)
- open plan buildings (1)
- open-air theaters (1)
- open-hearth furnaces (3)
- opera houses (5)
- operas & operettas (1)
- orangeries (2)
- ordinaries (taverns) (1)
- ordnance industry (12)
- ore docks (3)
- ore industry (6)
- organizations' facilities (2)
- organizations (120)
- orphanages (3)
- orphans (2)
- Orthodox churches (6)
- outbuildings (78)
- outfall sewers (4)
- overpasses (9)