"Oxygen" can refer to a few different things, so I will try to explain each of them a little bit. Oxygen is an element, which means that it is one of the ~100 different kinds of atoms that are found on earth, and is abbreviated "O". (An atom is the smallest unit of matter - everything is made up of atoms) Some other elements are hydrogen, nitrogen, copper, iron, and gold. When 2 oxygen atoms come close to each other, they form a bond, which forms an oxygen molecule, which we call diatomic oxygen, or "O2". Oxygen molecules, O2, are a gas under normal circumstances. This is the stuff in our air that we need to breathe to stay alive. (Air is a mixture of Oxygen gas, nitrogen gas, and a few others.)

Although when you hear someone say "oxygen" in everyday life, they are probably talking about the gas O₂, oxygen atoms are in lots of other materials as well. There are oxygen atoms in water, sugar, glass, rust, almost everything! But in each of those things the oxygen atoms are not in pairs, but rather bonded to other elemental atoms, like carbon, hydrogen, or iron.

I hope this clears things up a little bit, and if you have more questions, please ask!!

Oxygen is an element. Unless you have a nuclear reactor or something similar, you can't break it apart or split it into anything else. In nature, oxygen tends to exist in pairs of atoms, which we refer to as "oxygen" gas.

Oxygen is an element. Oxygen atoms have eight protons and eight electrons, and most oxygen atoms have eight neutrons as well, but some have more neutrons.

In the air, oxygen is a gas. One in every five molecules in the air is a molecule of oxygen - that is, two oxygen atoms bonded together.