Many scientists claim NDEs occur due to lack of oxygen in the brain, and hypoxia. However, this article has sources which challenge this.
http://skeptiko.com/critique-of-skeptics-guide-249/
I have also seen studies where patients who had NDEs had just as much oxygen as others who did not have life threatening problems. It seems to suggest that oxygen cannot be the answer. Oxygen deprivation seems to create weird hallucinations sometimes which are scary and painful.
Also, most of the time, people who have a lack of oxygen barely have any oxygen loss and they still report these same experiences. People don't even have to be near death to have them. Does this article prove NDEs are true?
http://skeptiko.com/critique-of-skeptics-guide-249/