It might really depend on how many carbs you’re eating, and how much you’re eating overall with your OMAD. I’d say after 12-16 hours I think the body starts going into ‘fat’ mode… but I can’t really provide anything for it.
Refer to this image: https://7sigmaphysiques.com/wp-content/uploads/2020/06/How-Long-Do-You-Have-to-Fast-Before-You-Start-Burning-Fat-Timeline.png
Basically after 12 hours of not eating, the fat burning ramps up. You see even more fat burning after 18 and 24 hours. So during Omad, you will experience fat burning for 11 hours of the day