An offensive rebound is when you get a rebound while your team is on the offensive end of the court (you or your teammates shoot).
You can get this naturally while upgrading your player no mater what position you play., but obviously it will be easier to get if you are a big man in the forward or center position. If you want to try and get this quickly without your main character, you can create a big man who is 7`3`and upgrade the stats as much as you can for offensive rebounds and you might as well do blocks at the same time. Give your guy as much wing span, height, etc... as you feel will be the best combo. You can also try a smaller guy at like 6`11`with more vertical and speed, yet I felt the traditional giant the easiest to knock this out quickest, since my main character was a PG
Then camp out near the rim, making sure to pop in and out of the key so you don`t get a 3 second violation. You can also try to get your own rebounds after your own shot misses by purposely missing close shots.
Play on longer quarters and easier difficulty to maximize your playing time and chances to get these achievements.