Potential side effects (mostly rare) of Hyperbaric Oxygen Therapy include, but are not limited to the following:

Barotraumas of the ear:  the most common complication

Round widow rupture:  a rare complication caused by the inability to equalize or vigorous performance of the forceful Valsalva maneuver

Sinus squeeze: may occur when upper respiratory symptoms are present.  Occurs when unable to equalize sinuses

Reverse Block:  Occurs during Decompression when gas is trapped in sinuses and/or middle ear

Visual Refractive Changes: Occurs after multiple treatments (20+) myopia may worsen and presbyopia may improve.  People may not need their reading glasses any more, but due to the nature of the lens of the eye which absorbs Oxygen rather than getting it through circulation, the vision changes for good or for worse, are only temporary and will within 6 weeks get back to pretreatment vision once HBOT has been stopped.

Numb Fingers: Numbness and tingling in the ulnar distribution with patients receiving multiple (20+) treatments.  This side effect is temporary and will resolve to normal within 6 weeks of the final HBOT treatment. 

Dental Problems: Pain caused by pressure changes affecting air spaces under fillings

Claustrophobia: Approximately 1out of 50 patients exhibit a level of confinement anxiety, rarely is it severe enough to require sedation.

CNS Oxygen Toxicity Seizures: Very rarely occurring less than 1% of 10,000 patient treatments excluding hypoglycemic seizures.  If you are hypoglycemic, or on insulin, be sure to eat prior to your HBOT treatment

Pulmonary Oxygen Toxicity: This has never been reported to have occurred during normal protocols where clinical treatments where O2 exposure is two hours or less.  However, this condition can occur when 100% O2 exposure exceeds 6 hours in a 24-hour period, and may exceed normal oxygen consumption if patient continues other forms of oxygen therapy beyond the chamber

"Oxygen Toxicity" K.K.Jain, Chp 6 Textbook of Hyperbaric Medicine

Please see the Scientific Abstracts and Articles link for more specific information and latest research.