What does OB mean in Japanese college baseball?

While OB stands for Obstructed Base Running in baseball, it is clear that there is no necessary connection here in OB Association

In fact, OB here is old boy, which means graduate/senior (in general)

So OB Association is the Old Alumni Association, and the College Baseball OB Association is the association of the old students of the baseball team of the school, which tends to become the oldest fans of the school club and give material support from the society. The OB Association is the association of old students of the school's baseball team, which tends to be the urn of the school's clubs and gives material support from the community

The corresponding OG Association is the Old Girl Association, which, of course, generally refers to the girls' alumni association

And the OB Association doesn't exclusively refer to the Boys' Association

The OB Association is the association of the graduates/seniors of the school.