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Oath: Written or oral pledge by a person to keep a promise or speak the truth.

Obiter Dictum: Remark by a judge in a legal opinion that is irrelevant to the decision and does not establish precedent . Often used in the plural, dicta. 
Objection: In a trial, a reason stated on the record by an attorney that a matter or proceeding is illegal. Making objections in open court is important for purposes of making a record for appeal.

Occupational Disease: An illness resulting from long-term employment in a particular type of work, such as those employees exposed to asbestos, who later develop cancer.

On a Person's Own Recognizance: Release of a person from custody without the payment of any bail or posting of bond, upon the promise to return to court.

Opening Statement: The initial statement made by attorneys for each side, outlining the facts each intends to establish during the trial.

Opinion: Written statement by a judge or court of the decision in a case which describes the law applied to the facts of the case and the reasons for the decision.

Oral Argument: An opportunity for lawyers to summarize their position before the court and also to answer the judges' questions.

Order: Written direction or command made by a court or judge, and not included in a judgment. See also decree.

Ordinance: Commonly, a regulation passed by a municipal legislative body.

Original jurisdication: The first court to which a legal dispute is referred.

Out-of-Court Settlement: An agreement reached between a plaintiff and a defendant to resolve a lawsuit privately and without a judge's authorization or approval.

Overrule: A judge's decision not to allow an objection. Also, a decision by a higher court finding that a lower court decision was in error.