- Oban Bay (s) - Pale ice blue. Chisholm, 1997.
- Oklahoma (s) - Crimson flake. 2002.
- Ola (s) - Pale mauve. Chisholm, 1999.
- Old Times (s) - Blue flush. 1976.
- Olive D (s) - Maroon stripe. Rowland, 2008.
- Oliver Cromwell (s) - Maroon. 2000.
- Olivia Faraday (g) - Mauve. Grayson, 2004.
- Olivia Grace (s) - Dark blue. 2012.
- Olympic Flame (s) - Salmon-cerise flush. Harrod.
- Omay - synonym of Happy Birthday.
- Orange Dragon (s) - Bright orange. 1958.
- Orange Flash (s) - Orange flake, white ground. 1960.
- Orange Perfection (s) - Orange/rose pink bicolour. Box, 1912.
- Orange Surprise (s) - Orange bicolour.
- Orchid (s) - Veined light lavender. 2008.
- Our Colin (s) - Mid blue. Chisholm, 2009.
- Our Harry (s) - Mid blue. Davis, 1987.
- Our Jenny (s) - Mid blue. 1993.
- Our Joyce (s) - Orange-pink. 1984.
- Ouse Valley (s) - Lavender. Tasker, 1985.
- Oxford Blue (s) - Mid blue. 2001.