text: difference between oba and baa? mlb.com lists a statistic for pitchers called "oba" which i believe means opponents batting average. yahoo.com lists a statistic calls "baa" which i think means batting average against.  for mike mussina the numbers are oba = .261 and baa = .222  so they must be calculated differently even though they sound similar. what's the difference? i'm sorry but oba doesn't stand for "opponents batting average", it stands for on-base against (oba).   which is calculated like this:  divide the total number of hits allowed, bases on balls allowed and batters hit by pitch by the total of the opposing batters' at-bats, bases on balls allowed, batters hit by pitch and sacrifice flies allowed. for example, if greg maddux allowed opposing batters 220 hits in 991 at-bats, allowed 27 walks, hit 7 batters with pitches and allowed 6 sacrifice flies, his opponents' on-base percentage would be .246 ((220+27+7)/(991+27+7+6)).     batting average against (baa): the batting average opposing hitters have against a pitcher.