O
- Oakland (1907–1931)
- Oakman-Hertel (see Hertel)
- Ogren (1915–1917, 1919–1923)
- Ohio (1900–1902) (see Packard)
- Ohio (1909–1912)
- Ohio Electric (1910–1918)
- Ohio Falls (see Pilgrim)
- Okey (1896–1907)
- Oldsmobile (1897–2004)
- Olympian (1917–1921)
- Omaha (1899)
- Omaha (1912–1913)
- Omar (see Browniekar)
- Only (1909–1913)
- Orient (1899–1908)
- Orlo (1904)
- Ormond Steamer (1904–1905)
- Orson (1910–1912)
- Otto (1910–1911; Ottomobile 1912)
- Otto-Kar or Ottokar (1902–1904)
- Otto-mobile (1899)
- Overholt (see Illinois)
- Overland (1903–1926, 1939)
- O-We-Go (1914)
- Owen (1899–1901)
- Owen (1910–1911)
- Owen Magnetic (1915–1922)
- Owen Schoeneck (see O-S)
- Owen Thomas (1908–1910)
- Oxford (1900)
Read more about this topic: List Of Defunct Automobile Manufacturers Of The United States