THE CREW HEARD A MUFFLED EXPLOSION & SAW FLAMES COMING FROM THE VENT NEAR SEAT 3-D WHILE ACFT WAS PARKKED AT GATE. LOCATION OF FIRE PREVENTED CREW FROM RETURNING TO COCKPIT TO NOTIFY CFR & ILLUMINATE EMERG FLOOR LIGHTING. PASSENGERS & CREW EVACUATED ACFT. SECOND OFFICER, LAST TO LEAVE, COULD NOT REACH REAR AIRSTAIRS & EXITED VIA EMERG WINDOW EXIT AFTER HAVING DIFFICULTY IN LOCATING AN EXIT BECAUSE OF SMOKE. A MECHANIC NOTED LOW PASSENGER OXYGEN SUPPLY DURING PREFLIGHT & REPLACED OXYGEN CYLINDERS. WHILE EXITING THE ELECTRICAL EQUIP BAY THE MECHANIC SAW A WHITE FLASH ENGULF THE OXYGEN SYSTEM FLOW CONTROL UNIT. HE ATTEMPTED TO HAVE CFR NOTIFIED OF THE FIRE USING A HAND HELD RADIO TO NO AVAIL. THE PASSENGER OXYGEN SYSTEM HAD 6 LOW OXYGEN QUANTITY MAINTENANCE WRITE-UPS DURING PREVIOUS 30 DAYS BUT WAS NOT 'FLAGGED' BY COMPANY AUTOMATED TREND ANALYSIS PROGRAM BECAUSE OF THE JOB TITLE OF THE PERSON ENTERING THE WRITE-UPS. INSPECTION OF DELTA'S FLEET REVEALED 35 OXYGEN SYSTEM LEAKS ON OTHER ACFT.