I tried doing a vegan diet while on OMAD, and I honestly don’t know if it’s possible for the average person. I say this not because it is entirely impossible—it’s very much possible—but it can be really difficult.
One of the main factors which account for its difficulty is that it can be very expensive to eat healthily, especially depending on your location. I currently live in a country where vegans and vegetarians probably make up for less than 1% of the population, combined. Because of this, the demand for and availability of vegan alternatives is extremely slim, resulting in both limited options and high costs.
There is no nutritional yeast (which is extremely important for vegan cuisine), no soy alternatives, no vegan protein supplements, and vegan dairy alternatives are absurdly overpriced, as are many other foods integral to a vegan diet. Getting enough carbs isn’t difficult, but meeting your daily quota for protein and healthy fats is much harder.
Like I said, it isn’t impossible, but you’d need a fairly above-average income if you want to eat vegan in a healthy way. Vegetarian is much more doable since you can include dairy and eggs in your diet.