Are there food that is unwise to eat or not recommended for eating?
As far as I'm aware, OMAD isn't really about restricting what you eat so much as it is about when you eat. While I don't think there are any things that you officially shouldn't be eating, there are certain kinds of foods which you should and shouldn't avoid as part of any healthy diet.
The first thing that comes to my mind are diet soft drinks, as they are notorious for causing food cravings. This is obviously not beneficial as the last thing you want are cravings when you can't eat for the next 23 hours.
I'd encourage eating foods that are high in fiber and protein, as these take a long time to digest and keep you feeling fuller for much longer.
I have read in another discussion that you can eat almost anything when you are in OMAD because the main principle for the effect is the no-food for a longer time. Eating once a day means you will have no food for 20 hours or so and that is the method of OMAD for losing weight. However, it is advised that we eat nutritious food especially vegetables and fish to at least have a semblance of a balanced meal.I think the whole point of OMAD is that you can continue to cut or maintain your weight while still eating anything you want to eat. I know people who have one meal of burger and chips and they are just fine.Personally I try to eat low carb, so I cut out my carbs and eat only protein and vegetables. I do this to accelerate my weight loss. Once I loose all the weight that I want I will be eating one meal of anything I want to eat, though I generally feel best when I'm eating low carb.