Ansys Employee

The mass fraction is relative to the whole (domain) or local (cell) fluid volume depending on your definition. However, O2 is a reactant, so I'm not sure why you're using it as a product; the typical two step reaction is (I may or may not have balanced the equations):

2CH4 + 3O2 -> 2CO + 4H2O

2CO + O2 -> 2CO2 

You wouldn't have a primary reactant as a product.