Oxidation of methane
Actually in $\ce{CH4}$ with slightly more electronegtaive carbon it is in $-4$ state. With $\ce{CO2}$ the oxygen is more electronegative oxygen carbon is $+4$. Therefore oxidation is also defined as addition/replacement of a more electronegative element.



Electronegativities $$\begin{array}{}\bf H&amp;2.2\\\bf C&amp;2.55\\\bf O&amp;3.44\end{array}$$