Hyperbaric Oxygen Treatment for Veterans With Traumatic Brain Injury

The goal of this blinded, adaptive, randomized, placebo-controlled clinical trial is to investigate the use of hyperbaric oxygen as a therapy to treat mild to moderate traumatic brain injury in Veterans and active military. The main questions it aims to answer are: * Does Hyperbaric Oxygen Therapy (HBOT) reduce neurobehavioral symptoms? (Aim 1) * How many HBOT sessions are needed to achieve a significant reduction in neurobehavioral symptoms? (Aim 2) * Does HBOT reduce posttraumatic stress disorder (PTSD) symptoms? (Aim 3) Exploratory objectives will explore if there are changes in: 1.) cognitive functioning using neuropsychological tests and the National Institutes of Health (NIH) toolbox, 2.) inflammation biomarkers in blood, 3.) microbiome in stool samples, 4.) electroencephalogram (EEG), 5.) sleep characteristics, and 6.) fMRI. Research will compare HBOT therapy to a placebo condition to see if HBOT works to treat neurobehavioral symptoms. The placebo condition is a chamber that remains unpressurized and has 21% oxygen. Participants will: 1. Complete baseline assessments to determine eligibility. 2. Attend 40 sessions of HBOT or placebo (normal air) within 12 weeks. 3. Complete questionnaires and interviews throughout the course of the study. 4. Complete a 2-week post treatment visit