There is a paper done on humans, where several types of magnesium were given...and blood then drawn and measured to see if magnesium levels changed (went up) after the dosing.

I don't know if cows with their 4 stomachs change the oxide type or not for absorption. But in humans the oxide is not appreciably absorbed. It moves on to the colon where it osmotically draws water into the lumen and improved stool passage. (laxative).

A typical MagOx for humans is 40% by weight magnesium. But it does not dissociate into free ionic form. So it is basically not useful for improving blood levels. It remains on the market and in doctor's therapeutic manuals, because no one has taken the effort to look at the newer information.
http://www.ncbi.nlm.nih.gov/pubmed/11794633

I am having good results using it in lotion form. I apply it to my wrists and legs where veins are near the surface of the skin.
It really helps me sleep quickly after applying at night.

This is a new study about magOx and bioavailability:
http://www.ncbi.nlm.nih.gov/pubmed/20862466
Ionized magnesium is preferred. There is a product called
Ionic Fizz which creates the ionic form from magnesium carbonate.

more:
http://www.ncbi.nlm.nih.gov/pubmed/1468087

So basically the organic chelates offer better oral absorption.
Chloride is also well absorbed according to some studies.
But any EXCESS in dosing of even these forms will be excreted and become laxative. There is a transport system in the bowel to move magnesium, and it can become saturated. One study suggests several small doses rather than one large one for this reason.