Offside: The right hand side of the horse.
Oldenburg: Breed of horse originally used as a carriage horse in Europe. Since the early twentieth century, the breed has been refined with infusions of Thoroughbred blood. Oldenburgs now excel in the dressage and jumping arenas.
On the Bit: A horse is said to be "on the bit" when he carries his head in a near vertical position and he is calmly accepting the rider's contact on the reins.
Orlov Trotter: Breed of horse originating in Russia. Used for light and heavy draught work, for pleasure and competition, and to improve other breeds of horse in Russia.
Overbent: An evasion where the horse tucks his head close in to his chest, reducing the rider's control. The horse is said to be behind the bit.
Overface: To present a young horse at a fence which is beyond his level of training, or beyond his physical capability.
Overo: Coat pattern seen in Paint Horses. Uneven splashes of white over the horse's belly, legs, neck and head. See also Paint Horse and Tobiano.
Overreaching: Where the toe of the hind foot catches and injures the back of the pastern or heel of the front foot. Occurs when the horse is galloping or jumping.
Oxer: Spread fence. Can be an ascending oxer - with the front rail lower than the back rail, or a square oxer (also known as a parallel), with front and back rail of the same height.