Honestly the calories creep up fast! I wouldn’t worry TOO much, unless you’re going OVER.
30g of cheese is about 120 calories
A nice heaping serving of meat can be several hundred calories (let’s say 500-600 for fun) but obviously depends what kind of fish or meat, & how it’s cooked, etc.
2 eggs can run you 140 calories
2 tbsp nut butter around 200 calories
1 Avocado can be up to 200-300 calories
Then add in some sauces and seasonings and bam already around 1500 calories they creep up quick. This is just an example of how calories can creep on quickly just put some random stuff together in my head to make this list hahaha! Hopefully people feel full more quickly while eating only OMAD, so it makes them eat LESS than if the were eating multiple meals a day! That way they might not even be eat that much!
One of my OMAD staples were also big fat Cobb salads, easily 1500 calories: I would do some lamb skewers, goat cheese, avocado, eggs, and just really load it up, was so easy to reach 1500 calories but you’ll get tired of chewing and you’ll get full before you finish
definitely a ridiculous amount of food in 1 sitting and usually hard to finish eating up to 1500 calories of GOOD/NUTRITIOUS/WHOLE foods in 1 meal because it’s so filling.
Then on the other hand, people can go to the drive through and get a meal that will run them 1500+ calories and not even satiate them 
Throwing around some examples
hope this helps