Oxygen Index : It is the minimum of oxygen in an oxygen -nitrogen mixture in which the material will burn(air contains 21 % oxygen) .

In the oxygen index test, temperature is approximately maintained at room temperature.

In actual practice during fire the extent of burning may be significantly influenced by the actual temperature involved. In order to overcome this, the oxygen index is measures over a range of temperature. From the results, a temperature index is obtained by extrapolation. As the relationship is non-linear,  extrapolation results are not accurate.

A more valid method is maintaining the oxygen concentration at 21% and varying the temperature, the temperature index being recorded as the  minimum temperature at which a material will support combustion following  its burning.