Oakhill (J. Monroe estate) [1 reference]

O'Brien, Richard [2 subentries]

“Observations and conjectures … on the … soil of Kentucky” [2 references]

Observations sur le gouvernement et les loix des Etats-Unis d'Amérique [1 reference]

O'Connell, Daniel [4 references + 1 subentry]

O'Connor, Thomas [3 subentries]

Odessa, Russia (now Ukraine) [1 reference]

Ohara, A.H. [1 reference]

Ohio [1 reference + 4 subentries]

Ohio River [3 references]

Old Sawney (Madison slave) [1 reference]

O'Neal, William [1 reference]

Onís, Luis de [1 subentry]

On Population [1 reference]

Ontario (U.S. sloop of war) [1 reference]

Opera [1 reference]

Opie, LeRoy [1 reference + 1 subentry]

Orange County, Va. [9 references + 10 subentries]

Orange Court House, Va. [3 references + 4 subentries]

Orange Order [10 references]

Oration, Delivered in St. Michael's Church … on the Fourth of July 1818 [1 reference]

Oration, Delivered in St. Philip's Church … on Friday, the Fourth of July, 1817 [1 reference]

Oration, Delivered, in St. Philip's Church … on the 5th Day of July, 1819 [1 reference]

Oration, Delivered July 5, 1819 [2 references]

Oration, Pronounced July 5, 1819 [2 references]

Ordnance Department [3 references]

Orford Hall (J. Blackburne estate) [1 reference]

Orleans Territory [1 reference + 1 subentry]

Ornithology [1 reference]

Orphan of Prague (play) [1 reference]

Orphan's Court, District of Columbia [1 reference]

Ortus Medicinae [1 reference]

Osage orange [1 reference]

Osborne, Richard [1 reference]

Otis, George Alexander [1 reference]

Outline of the Revolution in Spanish America [1 reference]

Ovid [1 reference]

Oxen [2 references]

Oxford University [1 reference]

Oxford, England [1 reference]

Oxford, Mass. (now Maine) [1 reference]