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.
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