Index: Adams Family Correspondence, Volume 12, O
- Odiorne, Mary Grindall (wife of Nathaniel)
- Odiorne, Nancy (daughter of Nathaniel)
- Odiorne, Nathaniel (of Unity, N.H.)
- Oglio River
- Old House (Peacefield, now Adams National Historical Park)
- Olmütz, Austria
- Orangist Party (also Court Party; Dutch)
- Orkney, Scotland
- Orléans, Louis Philippe Joseph, Duc d’ (Philippe Égalité)
- Orne, Rebecca Taylor (wife of Timothy)
- Orne, Timothy, Jr. (Salem merchant)
- Otis, Elizabeth Gray (1st wife of Samuel, Sr.)
- Otis, George (son of Samuel, Sr.)
- Otis, Harriet (daughter of Samuel, Sr.)
- Otis, Harrison Gray (Mass. representative)
- Otis, Mary Ann (daughter of Samuel, Sr.)
- Otis, Mary Smith Gray (wife of Samuel, Sr.)
- Otis, Samuel Allyne, Sr. (1740–1814, secy. of the Senate)
- Ottoman Empire
- Overton, Thomas (N.C. revenue inspector)