|This article does not cite any references or sources. (January 2007)|
An over-under or over/under (O/U) bet is a wager in which a sportsbook will predict a number for a statistic in a given game (usually the combined score of the two teams), and bettors wager that the actual number in the game will be either higher or lower than that number. For example, in Super Bowl XXXIX, most Las Vegas casinos set the over-under for the score of the game at 46.0. A bettor could wager that the combined score of the two teams would be either more than or less than that number. Since the combined score of that game was 45, anyone who had bet on "under" won.
Initial value 
The goal of a sportsbook is to have an equal number of bets on both sides of the over-under. In theory, this means that the manager could set the value at zero and then readjust based on either the number of incoming bets and/or events that influence the potential outcome. In practice, the initial value is based on both quantitative (e.g. win-loss record, average points per game, etc.) and anecdotal information (e.g. media reports, injury status of players, etc.).
Though this bet is most commonly made with the combined score of the two teams, many other statistics can be used, including:
- In American football, a player's or team's total rushing yards or attempts, down conversions (first or third), interceptions, completions, field goal percentage, etc.
- In basketball, a player's or team's total assists, blocks, turnovers, steals, etc.
- In baseball, a player's or team's total number of home runs, RBI, etc.
- In Association Football, The amount of goals scored in a game. The most popular market being Under/Over 2.5 goals