If oxygen is #"O"#, then what is #"O"_2# ?

It's true that Oxygen, can be termed as #"O"#. But one oxygen atom cant exist on its own, because its unstable.

Usually any atom needs 8 electrons in its outer orbit in order to stay stable.

But, the Oxygen atom has 6 electrons in its outer orbit. So, it needs two more electrons in order to make it 8. So, it makes a bond with another oxygen atom and shares 2 electrons each and becomes stable.

Its now represented as #"O"_2#

As you say, #O#, is the oxygen atom....i.e. an atom that has 8 nuclear charges....and 8 electronic charges whizzing round the nuclear core. Oxygen is promiscuously reactive, and even in its unreacted state....the STABLE form is the oxygen molecule....i.e. #O_2#. Some punters (including me) even refer to #"dioxygen"# or #"dinitrogen"# to emphasize this condition.

And a simple VESPER treatment gives the electronic structure of dioxygen is...

#:stackrel(ddot)O=stackrel(ddot)O:#...i.e. this is conceived to derive from atomic....#2xx:ddotO:#, the which atom has SIX valence electrons…

With respect to dioxygen, molecular geometry is LINEAR....electronic geometry with respect to each atom is TRIGONAL PLANAR....