"Offensive rebounds" refers to the act of a player from the team on offense getting possession of the basketball after a missed shot from their own team. Full definition
He led both teams in total rebounds and in offensive rebounds. (sbnation.com)
And even if he never has a really good jump shot, he can be extremely valuable as a guy that scores off cuts, in transition and on offensive rebounds. (sbnation.com)
But, you tend you give up lots of offensive rebounds if you keep attempting to block every shot. (sbnation.com)