O

Oestrus See estrus. 
Off feed Having ceased eating; without a healthy and normal appetite. 
Official production record Standard DHIA and DHIR records pertaining to milk production made under the supervision of an unbiased individual. Such records are used for management purposes (e.g., feeding, breeding, and culling), genetic evaluation (sire summaries), publicity, and sales. See DHIR.
Omasum The third division of the stomach of ruminants. Also called manifold, manyplies, and psalterium. 
On the hoof Designating a living meat animal. 
Open A term commonly used for nonpregnant farm mammals. 
Openness A quality in dairy cattle associated with length of body and width and length of rib. 
Organic nitrogen Nitrogen bound in organic compounds, such as protein or amino acids. Requires microbial decomposition prior to nitification as a plant nutrient.
Outcross Mating an individual to another in the same breed who is not closely related to it. 
Out of Terminology for mothered by in animal breeding. 
Ovariectomy The surgical removal of an ovary. 
Overconditioned An animal with excess flesh and patchy fat deposits, especially over the shoulders, back, hips, and thighs.
Owner-sampler (OS) An unofficial milk production record system, the records of which originate with the breeder (owner) through a local DHIA. This type of testing plan allows dairy farmers to weigh and sample milk from their cows. Records are intended for management use only and are not official.
Official The status given to DHI records that follow the Official DHI Rules. Data collection is supervised and the dairy herd is subject to verification tests. Official records may be published for advertising and promotional purposes.
Official DHI Used synonymously with DHI (see DHI). 
Official DHI Rules Basic and minimum standards to be followed uniformly and enforced in Official DHI and Official DHIR herds throughout the United States, ensuring accuracy, uniformity and integrity.
Oxytocin A naturally secreted hormone that is important in milk letdown and the contraction of smooth uterine muscles during parturition.