OMAD stands for "one meal a day". If you're too stupid to count calories (as most people are), then eating one meal a day can make it difficult to eat in a surplus. It's a fool-proof way for fools to lose weight, because eating once is easier to understand than "eat less". Also, if you force yourself to go hungry most hours of the day, you will eventually become better at ignoring hunger.

If you're not overweight, obese, and are fine with counting calories, there isn't really any point to it. Keto diets are also mostly bunk. Just get enough protein and calories throughout your day to meet your goals and move more.