Re:
the C2 and the O2 are different, but only slightly looks wise, the C2 is a "closed" system which means it has no expansion chamber which means that when the rotor/pads get really hot the fluid expands which causes your brake to bind on the rotor, the O2 is and "open" system which means it does have the expansion chamber, looks wise the difference is the C2 has a silver dial on the top of the reservoir. some of the seals etc should be the same.