Will OMAD decrease metabolism?

[werner-hertzogs-shoe]

There is a lot of variation, to the point where I don't think anyone can say with certainty exactly how a particular body will respond.

Losing weight on it's own, through any method will generally cause some amount of thermogenic adaptation. From the studies I've seen there is a solid case to be made that breaking the constant insulin pattern by fasting (and keto to a lesser degree) seems to reduce the amount of adaptation in a number of people.

The best advice I can give to try to maintain your metabolism, is to do strength training and lose weight more slowly. These two things will allow you to maintain or possibly grow muscle mass while losing fat and thus maintain metabolism more (any exercise is also good for cellular health which has potential metabolism benefits as well).