Discoveries of the beneficial cellular and biochemical effects have strengthened the rationale for the administration of hyperbaric oxygen therapy (HBO2) as an adjunctive therapy for the treatment of osteoradionecrosis (ORN). Malignancies, however, are considered a contraindication for HBO2 because of the possible tumor-promoting effects. The aim of this study was to examine the effects of HBO2 therapy on tumor weight, and to measure the progression of apoptosis and tumor cell proliferating activity in a cultured human oral cancer cell line. Twenty 5-week-old male NODscid mice underwent daily HBO2 of 2.5 atm abs, 90 minutes for 20 treatments. The control group, n = 20, did not undergo HBO2 and tumor weight, apoptosis index, and proliferating activity parameters were compared between the two groups. The results showed no significant differences (p < 0.05) in the whole-body weights, tumor weights, apoptotic index or proliferating activity index between the two groups. By using the apoptosis and proliferating activity assays which were better indicators of tumor cell growth than tumor weight alone, our results suggest that the clinical application of HBO2 does not promote the growth or proliferation of human oral cancer cells.