The Cleveland Browns superstar has been sidelined for the rest of the season after going down in the 1st quarter of the Oct. 25 game against the Cincinnati Bengals ... calling the situation "really f'n tough."

But, OBJ has vowed to bounce back better than ever -- and he's putting his money where his mouth is ... posting footage Thursday from a recent workout.

28-year-old Odell can be seen going through squats and catching drills ... and it's pretty impressive considering the dude had surgery just 3 weeks ago.

"Lord I swear I think I'm healed," Odell said on Wednesday ... as he walked comfortably out of the training facility.

It's been a tough time for Beckham -- who has had his fair share of injuries throughout his career.

The star receiver spoke out about his struggles in an Instagram post this week ... saying, "Ima be honest.... I been thru hell these last couple years n I just keep it pushin... this shxt ain’t for errbody!"