protein, fat, and fiber are the things to target if you’re looking to be able to sustain an OMAD style way of eating. Fiber helps fill you up in the short term, protein and fat help keep you satisfied over the next day. Its certainly more difficult to do that while vegetarian.
I’d probably focus around making stir frys or similar built spinach/greens, brussels sprouts, mushrooms, seitan, and nuts. Make it in very large quanities, and eat until full. Then have leftovers the next day, if necessary make some more.
If you’re still trying to lose weight, that one hour of calorie intake still needs to result in a caloric deficit.
I’ve been doing OMAD for three weeks and 11lbs down as of yesterday. I eat 800-1000 for my one meal. Usually eggs, cheese, and maybe bacon. I work out fairly hard every day too.