© 2020 SPST. We have previously demonstrated that photoresist removal rate is enhanced by adding a trace amount of O2 to the atmosphere in which H radicals are produced from H2 on a hot metal filament. In this case, not only H radicals but also O and OH radicals are produced. The populations of O and OH radicals are a few hundredth parts of that of H radicals, but these radicals must play important roles. It is not clear which radicals contribute more to the enhancement of the removal rate. We used He/O2 mixtures in this study, instead of H2/O2, to produce O radicals without co-producing H and OH to make clear the contribution of O radicals on the removal rate. The removal rate increased slightly with increasing the O2 additive amount when the filament was unheated. This may be caused by thermal oxidation. On the other hand, the removal rate with a hot filament decreased by the addition of O2. In short, the removal rate is not enhanced by O radicals. The enhancement in H2/O2 mixtures must only be ascribed to OH radicals.