alumnus; former member; old boy
Refers to a former member of a school, company, team, or organization. Often used in contexts like 大学のＯＢ (university alumni) or 会社のＯＢ (former company employee).
See also: オールドボーイ
彼はこの大学のＯＢです。
He is an alumnus of this university.
会社のＯＢが集まって同窓会を開いた。
Former employees of the company gathered and held a reunion.
Golf term; used when a ball goes outside the playable area. Also used metaphorically in other contexts to mean something is off-limits.
ボールがＯＢになった。
The ball went out of bounds.
He is an alumnus of the University of Tokyo.
その話題はこの会議ではＯＢだ。
That topic is out of bounds in this meeting.
ティーショットがＯＢになった。
His tee shot went out of bounds.
He left the company and became an OB (former employee).
He hit the ball out of bounds twice today.
ＯＢ is used for a former male member (old boy), while ＯＧ (old girl) is its female counterpart, and learners often mix up which letter applies to which gender.
Abbreviation of English 'old boy'. The golf sense is from 'out of bounds'.