During my medical at Biggin the doc said I'd failed for aircrew, because I had a broken nose (rugby injury).
"You'll have problems with an oxygen mask" he opined.
"Look", says I, "I'm wearing jeans and dessies, not a collar and tie, and I've been flying Army helicopters for the last six years as a crewman without once having to resort to oxygen".
It still took a phone call to the SAM at Middle Wallop to convince him.
Go for a second opinion.