The Oxygen Effect refers to the discovery (made in 1921) that living cells are significantly more damaged by radiation (X-ray, particle beams etc) in the presence of oxygen.
It's currently of particular sigificance to radiologists, who want to preserve cells undamaged (e.g. X-rays) - or when deliberately trying to damage cells in tumour radiation etc. The presence of oxygen can increase the rate of damage by up to 300%.
There are several theories which attempt to explain the effect - but no general agreement as yet.
The best known explanation of the oxygen effect is the oxygen fixation hypothesis developed by Alexander in 1962, which posited that radiation-induced non-restorable or “fixed” nuclear DNA lesions are lethal to cells in the presence of diatomic oxygen. Recent hypotheses include one based on oxygen-enhanced damage from first principles.. Another hypothesis posits that ionizing radiation provokes mitochondria to produce reactive oxygen (and nitrogen species), which are leakage during oxidative phosphorylation that varies with a hyperbolic saturation relationship observed with both the oxygen and nitric oxide effects