Hi Janet,
I think it depends on your goals but I'll start by saying a lot of folks don't necessarily count calories when doing OMAD. Others count things like carbs or look at their overall macros. While some make sure they are eating whole, real foods and not highly processed food.
Are you plant-based? If so, that may make it hard to get all the calories you'd like in one meal considering veggies and fruit are voluminous but low calorie so you end up getting full fast. At least, for me when I am eating plant-based I get full quickly like from a couple servings of roasted veggies and a couple apples. In that case, beans and grains may help.
I would say listen to your body. How long is your fasting window? Are you able to make it through it without feeling "true" hunger, dizziness, etc. I typically know I am eating enough because I don't get truly hungry until it's really close to my eating window.