There is no real, discrete temperature at which an oxidation reaction (or any other chemical reaction for that matter) starts.

Instead reaction rates are dependent on absolute temperature and follow [Arrhenius equation][1].

In short this means that at low temperatures (e.g. close to room temperature) reaction rates are usually (but not always) almost infinitesimally small and then increase as temperature is increased. **There is however no discrete temperature at which the reaction can said to 'start'**.

To get a better idea of this rate/temperature dependence, chemists will usually perform an Arrhenius plot.


  [1]: https://en.wikipedia.org/wiki/Arrhenius_equation