On base percentage

From BR Bullpen

On Base Percentage (aka OBP, On Base Average, OBA) is a measure of how often a batter reaches base. It is approximately equal to Times on Base/Plate appearances.

The full formula is OBP = (Hits + Walks + Hit by Pitch) / (At Bats + Walks + Hit by Pitch + Sacrifice Flies). Batters are not credited with reaching base on an error or fielder's choice, and they are not charged with an opportunity if they make a sacrifice bunt.