- off base (1865)
- official at-bat (1901)
- off-speed pitch (1954)
- official scorer (1872)
- old college try (1912)
- old timer (1872)
- Old Timers’ Day (1898)
- on-base average (1931)
- on-base percentage (1941)
- on-deck circle (1932)
- one base at a time (1868)
- one-base hit (1869)
- one-dimensional player (1974)
- one game at a time (1934)
- one-handed catch (1860)
- one-hitter (1913)
- One Hundred Thousand Dollar Infield (1911)
- one-two-three (1862)
- on the trademark (1887)
- opening day (1862)
- open stance (1921)
- opposite field (1902)
- opposite field hitter (1938)
- out by a mile (1892)
- outduel (1908)
- outfielder (1863)
- out in front (1926)
- out in left field (1922)
- outside corner (1886)
- overmanage (1896)
- overrun (1867)
- overslide (1886)
