The OEG concept takes advantage of the peculiarities of our binocular vision. A human brain takes image data from both eyes and seamlessly superimposes it. Generally, most of it comes from our dominant eye, with the other eye providing some additional detail and field of view.
An OEG tricks our vision into doing something a little different. Occluded means “blocked” or “obstructed.” You cannot see through an OEG. All it does is project a bright aiming point to your dominant eye that looks like it is fairly far away so your eye can still be focused on a distant object.
However, if both of your eyes are open and your other eye is focused on the target, the brain will superimpose the bright red dot from the dominant eye onto the image of the target from the nondominant eye. The important thing is to not try to focus on the dot. This method works for high-speed engagements where you keep your focus on the target. If you try to focus on the dot, it all falls apart.
OEGs do not work for everyone. Some people do not have very good binocular vision (many are born that way, and others have had some sort of an eye injury). This also does not work for people who are cross-eye dominant (i.e., right-handed with left-eye dominance) if they shoot with the dominant side.
Aside from these issues, OEGs are extremely fast and work great for center-mass hits at close range. For me, 25 to 30 yards are the practical limit for reasonably accurate OEG use, but I know of people who are successful with them out to around 100 yards.