Inclusion Criteria:  Aged 18-60 years Healthy or mild systemic disease (ASA 1-2) prior to COVID-19 Symptoms consistent with Long COVID for at least 12 weeks Diagnosed with Long COVID, PACS, PCS (ICD-10 U09.9) Working or studying prior to COVID-19 Documented informed consent according to GCP and national regulations  Exclusion Criteria:  Known pregnancy or positive pregnancy test in women of childbearing age ASA 3 or more from other cause than Long COVID Score above 70 in RAND-36 Role Limitation Physical Health (RP) or Physical Functioning (PF) Diabetes Diagnosed with hypertension prior to COVID-19 Contraindication for hyperbaric oxygen treatment according to local guidelines Participation or recent participation in a clinical trial with an investigational product Mental inability, reluctance or language difficulties that result in difficulty understanding the meaning of study participation