This is my magnesium thread:
http://neurotalk.psychcentral.com/thread1138.html

Magnesium oxide is the poorest least absorbed form you can take.
In fact so much remains in the bowel (this is because it does not ionize or dissolve in water) that it is used as a laxative today.

This post in that thread:
http://neurotalk.psychcentral.com/post20739-10.html
explains that in testing volunteers only about 4% is absorbed from magnesium oxide.

Magnesium chloride (SlowMag) or a chelate is a better choice unless you want a laxative.
In the US magnesium oxide is in Phillips tablets, a laxative.
It remains OTC for historical reasons, but newer products are far better. The amount absorbed from magnesium oxide 400mg is about 8-10mg which is the amount you get from an egg. This is calculated from the 400mg magoxide since only 200mg is actually the elemental value of the magnesium.
RDA for adults is around 350mg a day (elemental) from food.

This link helps you choose foods:
http://www.slowmag.com/

SlowMag was designed for doctors to prescribe, but it remains OTC in the US. It is available in US in generic products one of which is called Mag64.

Magnesium oxide is very inexpensive and still is in many mixtures with calcium. Don't count on its absorption therefore.
Its low price lures many people to it, but it is a very limited product. You can absorb far more magnesium by sitting in a bath tub with epsom salts. (6-8 oz in a large bathtub of LUKEWARM water.)