Moles
number of molecules divided by Avagadro's number (about 6*10^23 , Use Google if Necessary)
There are that number of molecules in a gram molecular mass.
Mass of a mole of O2 is about 32 grams which is about 6*10^23 molecules.
To say it another way mols = grams/molar mass
That formula uses smaller numbers for the same thing.