Encyclopedias, Almanacs, Transcripts, and Maps
- Otto Eduard Leopold von Bismarck
- Otto Graf Lambsdorff
- Otto Warburg
- Otto, Bodo
- Otto, Louise
- Ottoman Empire: France and Austria-Hungary
- Ou-yang Hsiu
- OUN/UPA
- Our Home is Russia Party
- Out Liers
- Outer Continental Shelf Lands Act (1953)
- outfangthief
- Over Mountain Men
- Over-The-Counter Securities Market
- Overman Act
- Overseers
- Owen Model Communities
- Owen, Chandler
- Owen, Wilfred (1893–1918)
- Oxenstierna, Axel (1583–1654)