A320 CREW OPERATED A FLT WITH ALMOST NO CREW OXYGEN.

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DURING PREFLT WE NOTED THAT CREW OXYGEN WAS 400 PSI. WE BOTH CHKED OUR MASKS AND REGULATORS AND THE CAPT SAID HE THOUGHT HIS WAS STUCK OPEN. WE DETERMINED THAT WE WERE BELOW THE MINIMUM REQUIRED PRESSURE AFTER CHKING THE ACFT FLT MANUAL. WE THEN DISCUSSED WHAT OUR OPTIONS MIGHT BE; WHEREUPON THE CAPT LEFT THE ACFT TO CONSULT BY PHONE WITH DISPATCH AND MAINT. THE CAPT RETURNED AND SAID THAT IT HAD BEEN DECIDED THAT WE WOULD FLY TO SJU WHERE BOTH OXYGEN AND MAINT WERE AVAILABLE. HE ASKED FOR MY OPINION AND I SAID THAT I FELT COMFORTABLE WITH THE DECISION SINCE IT WAS A VERY SHORT FLT AT LOW ALT AND PASSED RIGHT BY SAINT THOMAS ARPT. WE WERE DISPATCHED FROM SXM TO SJU AND COMPLETED THE FLT UNEVENTFULLY. AT SJU; CONTRACT MAINT REPLACED THE CREW OXYGEN BOTTLE AND INSPECTED THE OXYGEN SYS. WE THEN CONTINUED OUR FLT TO PHL. AFTER REVIEWING OUR SIT WITH SAFETY REPRESENTATIVES; I NOW BELIEVE WE SHOULD NOT HAVE DEPARTED SXM WITH LOW OXYGEN. THE ONE THING THE CAPT AND I HAD FAILED TO CONSIDER WAS AN EMER (SUCH AS SMOKE IN THE COCKPIT) WHICH WOULD HAVE REQUIRED OUR USE OF OXYGEN MASKS. IN RETROSPECT; I THINK OUR DISCUSSIONS HAD NARROWLY FOCUSED ON THE NEED FOR OXYGEN TO BREATHE AT ALT AND OVERLOOKED THE CONSIDERATION OF SMOKE IN THE COCKPIT. APPARENTLY; THIS WAS ALSO OVERLOOKED BY DISPATCH AND MAINT. I FEEL THAT WE USED GOOD CRM IN THAT THE CAPT AND I DISCUSSED THE SIT AT LENGTH AND THEN CONSULTED BOTH DISPATCH AND MAINT. MAYBE; IF THE FLT MANUAL HAD A WARNING OR CAUTION NOTE NEXT TO THE OXYGEN TABLE IT WOULD HAVE HELPED.