That’s pretty surprising that you managed to gain that much weight doing OMAD over 7 months, how many calories roughly were you taking in? If it’s not a crazy high number I would maybe visit a doctor to run test on hormones that effect metabolism. If you’ve got a thyroid issue especially, that could be part of the problem. 20lbs in 7 months equates to an average surplus of 333 cal a day, which is pretty hard to do OMAD in my experience.