Magnesium oxide is also a laxative so the real bioavailability is ~0% if it's triggering diarrhea. Magnesium (bis)glycinate is good, though it's formed by reacting magnesium oxide with glycine and often has substantial amounts of magnesium oxide remaining in it, so again if it's triggering diarrhea you're not absorbing any of it and need to find a different one.