What was found
(CAN) WHILE PERFORMING AN INTERIOR PREFLIGHT CHECK THE F/O PULLED THE CONTROL KNOB FOR THE OXYGEN SYS. THE KNOB WAS DIFFICULT TO PULL BUT THE PILOT CONFIRMED THAT THERE WAS FLOW AT THE MASKS. DURING THE FLIGHT THE PILOT AND CO-PILOT DONNED THE OXYGEN MASKS AND FOUND THERE WAS NO OXYGEN FLOW. UPON RETURN TO BASE MAINT CHECKED THE SYS AND FOUND IT SERVICEABLE. DURING THE NEXT FLIGHT THE PILOTS FOUND THE SAME PROBLEM WITH THE CONTROL KNOB. DURING TROUBLESHOOTING THE AC WAS COLD-SOAKED AND THE KNOB FOUND DIFFICULT TO PULL. THE CONTROL CABLE ASSY WAS REMOVED AND REPLACED. (TC NR 20071206003)