Notice that oxygen has 6 electrons (the circles with negative signs in them) orbiting its outermost ring. For an oxygen to be stable, it requires 8 electrons on its outermost ring. In order to do that, it must find another atom to share electrons with. Very often, oxygen atoms choose to share electrons with another oxygen atom, forming a big component of the air you breathe O2. O2 in plain english is like saying “two oxygen molecules got together to share their outer electrons and gain stability” like a weird kind of marriage.
