Why do enthalpy changes of combustion and formation not take into account oxygen?
By convention elements in their most stable polymorph at standard conditions have at standard conditions (298.15 K) a standard enthalpies of formation $\Delta{}_fH^\circ$ of zero (one reference).  This is why thermodynamics discerns C(graphite) from C(diamond), and C(fullerene), for example.  Hence on first view it looks like zeros in your example (graphite, Hydrogen, Oxygen) got forgotten; but they are not.