Eat slower, give your body time to ramp up the blood sugar and transition to digestion without it being a tidal wave. Think of how many minutes it takes you to eat, it’s likely only a couple. Then double or triple it and consciously space it out. Also, if you’re eating meat protein, chew more.
I am also keto/mostly carnivore and OMAD. You need more water. It takes the body quite a bit of energy expenditure to digest the calorically dense foods you are now eating. Make sure you are consuming enough electrolytes as well. 3-4 liters of water daily is optimal. If the increased hydration does not work, switching your meal time to later in the day may also help. I noticed this when I first started if I ate before “dinner time” and did OMAD I would want to have a nap, so I moved my eating time later and it helped but the key for me was definitely needing to consume more water.