Study aim: Investigating the effect of ozone therapy on pulmonary sequels in patients with COVID-19 and the resulting hypoxia Design: Clinical trial with control and intervention groups, with parallel groups, without blinding, randomized, phase 2 on about 100 patients. Block randomization method will be used for randomization. Settings and conduct: Place of study: Ozone therapy centers affiliated to BaqiyatAllah University located in Tehran and Najaf Abad. Study population: patients referred to the mentioned centers; Type of blinding: none Participants/Inclusion and exclusion criteria: Inclusion criteria: Filling out the informed consent form; post COVID-19 oxygen dependent patients. Exclusion criteria: hypersensitivity to ozone; Albumin less than 3.4; Bleeding and coagulation disorders; Platelets less than 60,000; Uncontrolled hyperthyroidism Intervention groups: Intervention group: The intervention of these patients will be with a therapeutic dose of 35-50 micrograms/ml with O2/O3 mixture based on the inflammatory factors and severity of pulmonary involvement at the time of referral. Control group: patients referred to the hospital/clinic who receive only standard treatment. Main outcome variables: lung CT scan reports ,saturation of blood O2