Does the oxygen mask have a reservoir bag?  If so, how large is it?

If it's a 1-liter bag, cut it off and replace it with a clean 40 gallon trash bag liner.  Fold the open end over a couple times with a 1" fold then tape the fold with duck tape for a gas tight seal. 

Then cut a small piece of one of the opposite corners a little smaller than the opening on the bottom of the mask. Stretch this over the opening on the mask and secure with electricians tape for a gas tight seal.  I call this contraption a "Redneck" reservoir bag.

Crank the regulator to the maximum flow rate (15 to 25 liters/minute), place the mask over your knee to block the escape of oxygen, and give the new "redneck" reservoir bag time to fill.  When the bag is full exhale fully away from the mask, (into the room), then place the mask over your face, inhale deeply and rapidly until your lungs can't hold any more then remove the mask and exhale forcefully into the room. 

Exhale until your lungs feel empty (they're not) then do an abdominal crunch like doing situps and hold the squeeze until your exhaled breath makes a wheezing sound for a couple seconds.  This will squeeze out another half to full liter of breath highest in CO2 content.  Then replace the mask to your face and inhale fully and rapidly.  This is called breathing at forced vital capacity tidal volumes...  It is also the most effective method of hyperventilating.

Continue repeating this sequence until the pain is completely gone.

If the mask has no reservoir bag... make the redneck reservoir ban out of the 40 gal trash bag liner only this time cut a 1/2" corner off one of the opposite corners and insert a 1/2" PVC pipe union or a nebulizer mouthpiece and secure with electricians tape for a gas tight seal.  Cut the oxygen tubing off next to the defunct mask and insert it into the other opposite corner of the redneck reservoir bag and seal with electricians tape.

Inhale from the PVC union and exhale into the room...

I've made redneck reservoir bags on a couple occasions and they work far better than any oxygen mask you can buy.  There is no resistance on inhalation or exhalation.  This helps make up for the additional effort required in the above breathing technique.

If you're using this breathing technique properly, two things will happen on the way to the fastest abort of a CH you've ever had.  The first thing is you will start coughing uncontrollably after one or two forced exhalations...  This is normal and it will last for 15 to 20 seconds until your lungs adjust to the breathing technique...

The second thing that happens is you will experience some or all of the symptoms of paresthesia - a very slight tingling or pricking of the fingertips, lips, and back of the neck.  You'll also likely experience a slight dizziness. 

All these symptoms are completely normal when using this breathing technique.  They're telling you that your system has pumped out CO2 faster than your body is generating it (through hyperventilation) and this results in respiratory alkalosis. 

As strange as this may sound, these symptoms are your friend.  They're the best indication you're hyperventilating sufficiently to abort your CH as fast as possible.

When the pain is completely gone, relax and inhale normally from the redneck reservoir bag to bring your total time breathing oxygen up to a total of 15 minutes.  In other words, if it takes 10 minutes to abort the attack breathing at forced vital capacity tidal volumes, add anther 5 minutes breathing normally.

Finally, contrary to some of the old wives tales you may have heard...  this method of oxygen therapy is very safe.  I used it up to 8 times a day at flow rates up to and above 40 liters/minute and higher for five years...  I'm still here at 67. 

I also have over 3000 hours flying Navy fighters on and off aircraft carriers...  All 3000 hours were spent breathing 100% oxygen from takeoff to landing or trap aboard ship.  The average mission length was nearly two hours and I can assure you, we were sucking down that 100% oxygen at flow rates up to 40 liters/minute and higher many times during each flight.  I always passed my annual flight physical with PA and lateral chest X-rays...

Moreover, all Navy and Marine Corps fighter and attack pilots continue to do this today and have been breathing 100% oxygen on all flights from take off to landing since 1943... shortly after US engineers cockaroached the oxygen regulator design from a Bf-109 that landed in the UK by mistake. 

Our astronauts also go on 100% oxygen for suited operations for up to four hours at a time so this is a very safe method of oxygen therapy.

Take care and keep us posted.

V/R, Batch