Even diet soda is not recommended when you are wanting to lose weight. Although it does not have all of the sugar (and extra calories and carbs) of regular sodas, it tricks your body because it tastes sweet; so it still can cause an insulin response, and this is what we are trying hard not to do when we are fasting .
When you are having your meal of the day, if you are not restricting your foods, then you could have soda along with the meal; but even then it is not a healthy choice.
What I really like is plain carbonated water, or mineral water, which is even better and healthier, although more expensive. I have a SodaStream, and I use that along with plain filtered water to make my “fizzlated water” (as I call it), and then I sometimes add just a bit of lemon slice to the glass to give it more of a tangy flavor.
Here is one of the blog articles that Jimmy has written about which drinks work best for us when we are trying to get healthier and lose weight with OMAD way of eating.
https://omaddiet.com/omad-diet-liquids/