Hyperbaric Oxygen Therapy for Ulcerative Colitis (HBOT-UC)

Inclusion Criteria: * Participants with known or newly diagnosed UC who require hospitalization for an acute moderate to severe flare * Age 18-85 * Able to fully participate in all aspects of the trial * Consented and able to receive first HBOT session within first 48 hours of initiation of intravenous steroids * Agreement to not participate in another trial for the duration of the active intervention period Exclusion Criteria: * Received hyperbaric oxygen therapy either as part of standard of care or through a clinical trial prior to enrollment * Complication requiring urgent surgical intervention * Requirement for new start of a biologic or small molecule during the hospitalization prior to randomization and/or anticipated requirement for rescue medical or surgical therapy within 48 hours of randomization * Toxic megacolon * Inability to receive intravenous steroids * Historically failed or been exposed to 4 or more classes of advanced therapeutic options * Known or suspected diagnosis of Crohn's colitis, indeterminate colitis, ischemic colitis, radiation colitis, diverticular disease associated with colitis, microscopic colitis or infectious colitis * Received any investigational drug within 30 days * Clinically significant cardiac, renal, neurological, endocrine, respiratory or hepatic impairment that increases the risk for HBOT toxicity * Women who are pregnant or nursing * Unwillingness to complete course of HBOT