Oxidative induction time (OIT) is defined as the time to the onset of oxidation of a test specimen exposed to oxygen at an elevated isothermal test temperature. In the pressure differential scanning calorimetry embodiment of this test, the test specimen is often exposed to the oxidizing atmosphere as the apparatus is heated from ambient to the isothermal test temperature. This creates a bias in the measurement due to undetected oxidation on heating. An expression, based upon the Arrhenius equation, is derived and then numerically integrated to obtain an estimate of the bias introduced into the OIT measurement. The bias is dependent on the activation energy of the reaction and on the heating rate. It is found to be less than 1.2 minute for the most common heating rates, and is less than 3 minutes for the most extreme sets of experimental conditions. The bias is small when compared to experimental repeatability of the OIT measurement and to the mean of OIT values. For this reason, it may be ignored in all but the most extreme cases of low activation energy, very slow heating rates, very low OIT values and high test temperatures.