| Oaks |
 | A stakes event for three-year-old fillies (females). |
| objection |
| Claim of foul lodged by rider, patrol judge or other official after the running of a race. If lodged by official, it is called an inquiry. |
| oblique (fracture) |
| Fracture at an angle. |
| OCD lesion |
| A cartilaginous or bony lesion that is the result of a failure in development. |
| odds-on |
| Odds of less than even money. |
| off side |
| Right side of horse. |
| off-track betting |
| Wagering at legalized betting outlets usually run by the tracks, management companies specializing in parimutuel wagering, or, in New York State, by independent corporations chartered by the state. Wagers at OTB sites are usually commingled with on-track betting pools. |
| official |
| 1) Notice displayed when a race result is confirmed. 2) Used to denote a racing official. |
| oiled (oiling) |
| Administration of mineral oil via nasogastric tube to relieve gas or pass blockage. Preventative procedure commonly used in long van rides to prevent impaction with subsequent colics. See colic. |
| on the bit |
| When a horse is eager to run. Also known as "in the bridle." |
| on the board |
| Finishing among the first three. |
| on the muscle |
| Denotes a fit horse. |
| on the nose |
| Betting a horse to win only. |
| open fracture |
| See compound fracture. |
| open knee |
| A condition of young horses in which the physis of the knee has not closed; an immature knee. Often used to describe the status of the physis immediately above the knee and is an indicator of long bone growth in two-year-olds. |
| osselet |
| See arthritis. |
| osteoarthritis |
| A permanent form of arthritis with progressive loss of the articular cartilage in a joint. See degenerative joint disease. |
| OTB |
| Abbreviation for off-track betting. |
| out of the money |
| A horse that finishes worse than third. |
| over at the knee |
| A leg that looks like it has a forward arc with its center at the knee when viewed from the side. |
| over-reaching |
| Toe of hind shoe striking the forefoot or foreleg. |
| overcheck |
| A strap that holds the bit in place. |
| overgirth |
| An elastic band that goes completely around a horse, over the saddle, to keep the saddle from slipping. |
| overland |
| Racing wide throughout, outside of other horses. |
| overlay |
| A horse going off at higher odds than it appears to warrant based on its past performances. |
| overnight |
| A sheet published by the racing secretary's office listing the entries for an upcoming racing card. |
| overnight race |
| A race in which entries close a specific number of hours before running (such as 48 hours), as opposed to a stakes race for which nominations close weeks and sometimes months in advance. |
| overweight |
| Surplus weight carried by a horse when the rider cannot make the required weight. |
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