Objective: Osteoarthritis (OA) is a chronic degenerative musculoskeletal disease and one of the leading causes of chronic musculoskeletal pain, often affecting the knee. Despite intra-articular (IA) injectable hyaluronan (HA) preparations are widely used in the treatment of this debilitating condition, only a few data about their efficacy have been reported so far. Therefore, the use of HA in OA management is not universally recommend in clinical practice. Oxygen-ozone (O 2 O 3 ) therapy can be employed in several pain-related conditions and diseases. However, both its efficacy and safety for IA therapy in knee OA have never been explored. Here, we evaluated the reliability of knee IA therapy with O 2 O 3 compared to IA HA in chronic knee OA.Methods: A total of 42 consecutive chronic OA patients with radiological diagnosis of knee OA were prospectively enrolled in this single-blind, controlled study. After randomization, all patients underwent IA therapy with O 2 O 3 or HA (q1wk) for 4 weeks, with additional 4 weeks of follow-up. Examination of the adverse events occurred during the whole study was performed. To measure knee function and pain, visual analogue scale (VAS), Oxford knee questionnaire (OKQ), and 12-item short form survey (SF-12) were administered. EuroQol five dimensions questionnaire (EQ-5D) was used to assess patients' quality of life.Results: No significant difference in adverse events occurrence was observed. Knee IA treatment with O 2 O 3 showed shorter reduction of pain compared to IA HA. VAS score decreased in both groups during the treatment period (p<0.001), while OKQ score significantly increased (p<0.001). SF-12 and EQ-5D scores were comparable between the two groups of patients.Conclusions: Despite IA administration of O 2 O 3 and hyaluronan are comparable treatments in knee OA both in terms of safety and quality of life improvement, the latter shows longer times of pain reduction.