O
- Oak trees (3)
- oak trees (3)
- Oatmeal Mill ()
- Obelisks (7)
- obelisks (7)
- Observatories (11)
- observatories (11)
- Observatory ()
- Obstetrical Building ()
- Octagonal buildings (11)
- octagonal buildings (11)
- Octagonal towers (2)
- octagonal towers (2)
- Office & Warehouse ()
- Office buildings (236)
- office buildings (236)
- Office Building ()
- Office parks ()
- Office Space ()
- Office/Finishing Rm. ()
- Office/Lab/Shop ()
- Office/Warehouse ()
- Officer Quarters ()
- Officer Residence ()
- Officer Service Club ()
- Officer's Family Hsg ()
- Officer's Family Qtr ()
- Officer's House ()
- Officer's Quarters ()
- Officers' clubs (11)
- officers' clubs (11)
- Officers' Club ()
- officers' quarters (384)
- Officers' quarters (384)
- Officers'Quarters ()
- Offices-Labs ()
- Offices/Supply Cent ()
- Offices (590)
- offices (590)
- Office (236)
- official residences (8)
- Official residences (8)
- oil houses (13)
- Oil Refinery ()
- Oil Storage & Supply ()
- Oil Storage House ()
- oil storage tanks (6)
- Oil Storage (6)
- Oil Tank Storage ()
- Oil Unloading Dock ()
- oil wells (9)
- Oil wells (9)
- Oil-fired power plants ()
- Old Baldwin Hme Site ()
- Old Paper Mill ()
- OLST ()
- Olympic ()
- one-room schools (16)
- One-room schools (16)
- onion domes (1)
- Open Hearth (3)
- Open plan buildings (1)
- open plan buildings (1)
- open spaces (8)
- open-air theaters (2)
- Open-air theaters (2)
- open-hearth furnaces (3)
- Open-hearth furnaces (3)
- opera houses (8)
- Opera houses (8)
- Opera House ()
- operas & operettas (1)
- operating rooms (1)
- Operations Building ()
- Operations (14)
- OPOT ()
- Orangeries (2)
- orangeries (2)
- Orangerie ()
- Ordance Mainten.Shop ()
- ordinaries (taverns) (5)
- Ordinaries (taverns) (5)
- Ordinary (2)
- ordnance industry (12)
- Ordnance Magazine ()
- ordnance testing (2)
- Ore chutes ()
- ore docks (3)
- Ore docks (3)
- ore industry (12)
- Ore Loading Dock ()
- Ore Mining ()
- Ore pocket dock cplx ()
- Ore pocket dock ()
- Ore Processing ()
- Oregon Caves ()
- Organ Pipe Cactus ()
- Organ Works ()
- Organizations' facilities (1)
- organizations' facilities (1)