Mg citrate has been reported to have greater bioavailability. The
bioavailability of MgO has generally reported to be poor. OTOH, it has been
effective in some clinical trials and is generally cheaper than the other
forms.
Lindberg JS, Zobitz MM, Poindexter JR, Pak CY. Magnesium bioavailability
from magnesium citrate and magnesium oxide. J Am Coll Nutr. 1990
Feb;9(1):48-55.
Firoz M, Graber M. Bioavailability of US commercial magnesium preparations.
Magnes Res 2001 Dec;14(4):257-62.