O
- O'Neal House (4)
- O'Neal, Clara Belle (1875-1879) (1)
- Odd Fellows Cemetery (1)
- Odd Fellows Hall (17)
- Odd Fellows' Halls (4)
- Odd Fellows' Hall (17)
- Oil houses (3)
- Olano, Mrs. Frank (1)
- Onions (1)
- Opium paraphrenalia (1)
- Opium tins (1)
- Orchestra concerts (1)
- Ore buckets (1)
- Ore carts (1)
- Oregon short lines (1)
- Organs (3)
- Oropa Santuario, Italy (3)
- Oropa, Italy (1)
- Outbuildings (29)
- Outhouses (11)
- Outlying Areas (38)
- Ovens (3)
- Overshoe liners (1)
- Owyhee, Nev. (2)