- 2 Ornate (typeface group)
- 5 Ornithological illustrations
- 2 Ornithologists
- 1 Ornithology
- 11 Orphan Asylum (Philadelphia, Pa.)
- 48 Orphanages
- 14 Orphans
- 5 Orphans (Sculpture)
- 24 Orsay, Alfred Guillaume Gabriel, comte d’, 1801-1852
- 2 Orthodox churches
- 3 Orthodox Street
- 2 Osage
- 1 Osage Avenue
- 1 Osborn Manufacturing Co.
- 5 Osborn Street
- 2 Oscawana Corners, New York
- 8 Oscawana, New York
- 1 Osceola Mills (Cope, Thomas and William)
- 1 Ossian
- 2 Oswego, New York
- 1 Otis Brothers & Co. (New York, N.Y.)
- 2 Otsego Street
- 4 Otters
- 2 Otto Eisenlohr & Bros.
- 1 Otto Gerdau (Firm)
- 20 Ottoman (style)
- 23 Ottoman Empire (empire)
- 3 Ouija boards
- 1 Our House Lodgings
- 5 Our Mutual Friend
- 5 Out in the rain (Sculpture)
- 6 Outbuildings
- 27 Outdoor advertising
- 1 Outdoor furniture
- 3 Outdoor sculpture
- 2 Outer space
- 1 Outerwear
- 2 Outgate, England
- 2 Outlook
- 4 Ouvry, Frederic, 1814-1881
