Oxygen Detection

[O] denotes the concentration of oxygen in the particular substance. So you would not only detect the presence of oxygen, but also how much is present.
This symbol is also used for "Oxygen Radical" or an atom of oxygen unpaired.

[O] means "oxygen"

media are there to indicate the concentration of the ion or molecule, in this case, oxygen.

I think the other issue is that with the elemental analysis there is no way to detect oxygen, rather it is learned by testing for all other items first. If the total mass is not 100%, the rest must be oxygen.