I do supplement magnesium for probably 2 years now, way before OMAD. Mainly to prevent/stop migraines. I found, that magnesium citrate gives me diarrhea even in small amounts such as 300 mg (of elemental magnesium from magnesium citrate), and I try to take minimum 600 mg a day. I can easily take that and double (if headache comes) without problems as magnesium glycinate, malate or taurate. I heard, that threonate is great too, however price is way to high. I used to get it from Innovix labs in capsules, but now use Seeking health malate powder and stuff it into capsules myself. I was hoping to use powder, but it tastes like laundry detergent...
There is a significant amount of magnesium found in pineapple juices. And this can be a good supplement instead of taking separate doses of magnesium. I have also read lately that magnesium aids in combatting depression. This is a welcome piece of news because people can now have the option to take mental-health-related meds via a refreshing drink of one's favorite pineapple juice.
I haven't tried taking magnesium supplement while doing omad. I'm also curious like you, magnesium provides many health benefits, both physical and mental. I think the best way to ensure that your body has the magnesium you need is to eat a diet high in magnesium-rich foods such as vegetables, nuts, legumes and whole grains. But if your diet is lacking in magnesium, you may need to take a daily supplement. Unless you have a medical condition that makes it hard to absorb magnesium, you should be able to get enough by eating the right foods. You should consider managing your diet before you start taking supplements
If you decide to take a supplement I suggest selecting one that contains a form of magnesium that is easily absorbed.