Oaks: A stakes race for 3-year-old fillies

Objection: interference complaint made by a jockey or trainer

Off Billet: the strap (or straps) on the off side that buckle one end of the girth in place

Off Side: the right-hand side of the horse; the off fore is the front right leg

Off The Board: describes the betting action on a horse which is being very heavily bet

On Deck: next to go in the pen

One-Paced: a description of a horse with acceleration

One-Run Type: a horse which tends to lag toward the back of the pack during the early stages of a race before mounting a late run

On the Bit: when a horse carries his head near vertically, with his mouth slightly below the rider's hand, he is said to be 'on the bit'

On The Bit: when a horse is eager to run

Open Joint: joint opened by a penetrating wound

Open (1): competition available for professionals, nonpros, amateurs, and youth. Anyone can enter

Open (2): not pregnant; a term for the outmoded procedure of reaching into a mare's vagina prior to breeding to open her cervix

Oriental Horse: a horse originating from Central or West Asia

Outrider: the person who rides the horses that accompany the race horses to the post and catches any horses who get loose

Overnight Race: a race where entries close a specific number of hours before running (such as 48 hours), as opposed to a stakes race where nominations close far in advance

Overreach: an injury which occurs when the horse strikes into the heel area of the front foot with the toe of its hind foot

Over-Stride: if the rear foot of a horse passes the track of the same side front foot as it is set down, the distance between the front of the front track and the rear of the rear track is the amount of over - stride

Overweight: surplus weight carried by a horse when the rider cannot make the assigned weight

Outside: in a pen, the side of the horse toward the fence

Ovary: one of the pair of ovum (egg)-producing glands of the female, which also produce sex hormones

Overo: a Paint or Pinto coat pattern of spots that are irregular, scattered, or splashy. The horse usually has a large white facial marking as well

Ovulation: when the follicle ruptures and the ovum, or egg, is released into the oviduct

Ovum: egg

Oxer: parallel bar-type fence with two rails. A square oxer has even rails. A step or ascending oxer has a lower front rail

Oxytocin: the portion of the posterior pituitary hormone which causes milk letdown and contraction of the uterus during foaling

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