Why are you supposed to eat at maintenance or a surplus on OMAD? You are eating for that day so shouldn’t it be a 500 calorie deficit regardless?
If you wanna loose weight you need to be in a caloric deficit. Speaking from personal experience, if you eat at caloric surplus you will gain weight on OMAD.
The benefit of OMAD is that’s it’s quite difficult to consume a huge caloric surplus in a single meal.