I agree that certain cats just can't free feed without gaining too much weight. Your Oscar looks very similar to Cirrus and Cirrus is also semi-feral and his mom was feral. When I first adopted Cirrus he was overweight. He was so stressed out that he'd eat way too much. I then slimmed him down to a very good weight. Now he's a heavy boy again but at least he's much happier now than he was with me. Good luck with the new slimming diet.