They were made inside red giant stars and thrown out of them in a supernova . The shell model explains why oxygen atoms can make 2 covalent bonds . For example when 1 oxygen atom joins with 2 hydrogen atoms they form water molecules as well as many of the molecules of life . We can think of oxygen atoms as a ball with 2 arms or as a shell with 2 holes. In models oxygen atoms are usually colored red.
Oxygen atoms like to gain electrons to give themselves a hard outer shell.
Oxygen is therefore an oxidizing agent , or electron acceptor. For the size of oxygen atoms, click here .