Magnesium oxide is claimed by some to increase the oxidative stress and is harmful to the body because it produces free radicals.

The utilisation of Mg oxide is indeed poor (except when used in a colloidal system like liposomes, where the best utilisation is of Mg oxide. Such Mg oxide has also been found to be better than Mg-bisglycinate in terms of its utilization) [1]

Mg oxide does not actually impair our antioxidant system – it improves it. Even if it can convert to free radicals and thus increase their number, it can still neutralise more free radicals, so overall Mg oxide has an antioxidant effect. [2,3]

I found two studies on the free radical enhancing and free radical reducing (antioxidant) effects of Mg oxide. One study looked in a separate experiment at how much it increases free radicals and how much it reduces them: increased them by 25% and decreased them by 77%[2] So, in combination, we can expect an antioxidant effect. This is supported by a later study where the antioxidant effect of Mg oxide in a nanocollide system was tested in combination with vitamin C and a polyphenol-rich extract of the Moringa plant. Mg oxide had a significantly stronger antioxidant effect than the polyphenol-rich extract, while vitamin C had about the same effect.[3]

Therefore, Mg oxide will not cause free radical stress to your body; this is an unfounded scare. Even if you increased the number of free radicals, that would not be a problem, because they play a very important role in the functioning of the body and it is only a constant excess of them that is problematic, as is a constant low level. Above a certain dose, all antioxidants act as pro-oxidants, i.e. they enhance free radicals. 20 years ago we thought that free radicals were only harmful, but since then we have found that this is not quite the case – they are also important and healthy to a certain extent. 

However, I can give you one reason not to take oxides: due to their poor utilisation, Mgoxide or other oxides are present in the intestinal tract for a longer period and in greater quantities than a more utilizable form. This can result to the following: if there so-called biofilm layers in the gut, Mg, Ca and other minerals/trace elements can be incorporated into them, strengthening the biofilm layers, which is built by the opportunistic ("bad") bacteria that lurking there to protect themselves. If you have an upset gut flora or digestive problems, there is a good chance that you may have biofilm layers in your gut, so it's a better idea to take more bioavailable mineral/trace mineral forms. Of course, this biofilm-strengthening effect is not specific to oxide forms, but to all poorly utilized forms. The significance of this problem is debatable, but it is worth mentioning