Magnesium oxide carries the most elemental magnesium per unit weight of the common forms, about 60% by molecular weight, but the least of it is absorbed.
In a comparison of four US commercial preparations, fractional absorption of magnesium oxide measured 4%, the lowest of the four (Firoz 2001, PMID: 11794633).
Most of a dose stays in the gut, where it draws in water and supports bowel regularity through an osmotic effect.
It is a poor choice for supporting systemic magnesium status: two controlled trials found it did not raise urinary or plasma magnesium relative to placebo or to magnesium citrate.
It is also the cheapest form, which is why it dominates products labeled simply 'magnesium'.
If a label says 'magnesium' without naming the form, it is almost always oxide. Our assessment: an ingredient for bowel regularity, not for magnesium repletion.