Magnesium oxide is a very cheap form of magnesium to make, it bioavailability is very low ... mag chloride is a decent form of magnesium, and there are plenty of other forms that you body well may like (I know from experience that different forms of magnesium work for some and not others).
You may want to look at Ultra Mag from source naturals, also doctor's best has a magnesium that I think is called bioavailable or high absorption magnesium...
I like the previous testimony on the transdermal, never tried that