Pitcher On Base Percentage is a measure of how often the batters a pitcher has faced reach the base.
It's calculated as:
OBP = Hits Allowed + Walks Allowed + Hit Batters/Opponent At Bats + Walks Allowed + Hit Batters + Sacrificed Hits Allowed
*Qualifying pitchers only
More about Pitcher On Base Percentage Leaders
Pitching opponent on base percentage leaders are those who allow the fewest number of plate appearances against them to result in a batter safely reaching base, either via a hit, walk, hits batsman, or catcher’s interference. Any pitcher allowing an opponent on base percentage under .300 is an above average pitcher, and the leaders for any given year will generally allow an opponent on base percentage of around .250 (with the truly elite pitchers sometimes allowing even fewer batters to reach base). While overall on base percentage is slightly lower in the National League, the leaders in opponent on base percentage are generally comprised of elite pitchers from both leagues.