Although topical oxygen therapy (TOT) is a promising treatment for chronic wounds, its clinical efficacy and safety remain to be rigorously established. We conducted a two-arm randomised controlled trial to evaluate the efficacy and safety of TOT for treating chronic wounds by a commercially available portable continuous diffusion of oxygen (CDO) system. Eighty-eight patients were allocated to the TOT (n = 44) or moist wound therapy (MWT) (n = 44) group for a 28-day intervention period, followed by standardised MWT for unhealed wounds until the 12-week endpoint. Eighty-eight patients were allocated to either the TOT group (n = 44) or the standard MWT group (n = 44) for 28 days (or until wound closure) and were followed for up to 12 weeks. Wound area, depth, pH, healing rate and healing time were assessed weekly for 28 days or until 12 weeks. Any adverse event was observed at the same time. At day 28, the TOT group demonstrated significantly greater reductions in wound area and depth compared with the MWT group (p < 0.05). The wound bed pH in the TOT group was lower than the MWT group at day 14 and 28. Although the healing rate was higher in the TOT group than in the MWT group at day 28 (45.5% vs. 11.4%, p < 0.001), the healing rate in both groups was similar at week 12 (95.5% vs. 90.9%, p = 0.536). The healing time of the TOT group was shorter than that of the MWT group at week 12 by 13.5 days (95% CI: 6.74–15.40; p = 0.004). No TOT-related adverse events were reported. These findings indicate that portable TOT can significantly accelerate wound healing, particularly by improving wound bed pH that could facilitate subsequent healing processes in patients with chronic wounds.