Inclusion Criteria:

1. Age ≥18 years;

2. Positive COVID-19 test by nasopharyngeal swab RT-PCR (reverse transcription
polymerase chain reaction) test, antibody or antigen tests at least 3 months prior
to randomization; OR Presumed COVID-19 assessed by the site investigator (no
positive COVID-19 test) with acute illness after October 15, 2019.

3. Patients should be treated with standard of care therapies (as discussed in the
study manual) for at least 4 weeks prior to entry into trial.

4. Lingering COVID-19 symptoms beyond 3 months from onset of acute COVID and symptoms
have lasted at least 2 months. The onset of COVID is considered the earliest of two
dates: the date of positive test or the date of first symptoms;

5. Lingering symptoms from COVID-19 present at the time of randomization. "Lingering
symptoms of Long COVID" must include self-reported cognitive dysfunction symptoms.

6. Female patients of childbearing potential (as assessed by the overseeing
Investigator) who are sexually active must agree to practice true abstinence or use
effective methods of contraception while on study treatment. Effective methods of
contraception must be discussed and approved by the overseeing Investigator.

7. Must be able to provide informed consent and both willing and able to comply with
study requirements.

8. A confirmed ability to travel to one of the three sites where HBOT or sham can be
administered.

Exclusion Criteria:

1. Patients who had mechanical ventilation or extracorporeal membrane oxygen (ECMO) for
COVID-19;

2. Current end-organ failure, organ transplantation, or current hospitalization in
acute care hospital;

3. Contraindications to all of the study interventions;

4. Co-enrolment in another interventional trial (co-enrolment in an observational study
is permitted);

5. Currently pregnant or breastfeeding.

6. Pneumothorax

7. Poorly controlled seizure disorder

8. Chronic sinusitis

9. Chronic or acute otitis media

10. Major ear drum trauma.