What was found
FAR 135.415 REPORT. PILOT REPORTED DEPARTING, WHEN THEY PERFORMED O2 MASK CHECK, CLIMBING THROUGH 18,000' OXYGEN SYS INDICATOR DROPPED TO 0 PSI & THEY RETURNED. FOUND THE ON & OFF SELECTOR KNOB FOR THE O2 BOTTLE INDICATING THE ON POSITION. DURING FURTHER TROUBLESHOOTING THE CAP ON THE SELECTOR KNOB WHICH COVERS THE ATTACH BOLT WAS REMOVED SHOWING MULTIPLE CRACKS IN THE KEYED BOSS OF THE SELECTOR KNOB WHICH ROTATES THE REGULATOR ON & OFF SHAFT. THIS ALLOWED THE KNOB TO BE ROTATED INTO THE ON POSITION WITHOUT ROTATING THE REGULATOR SHAFT COMPLETELY TO THE ON POSITION. R & R THE OXYGEN BOTTLE & REGULATOR ASSEMBLY & OPERATIONS CHECK WAS SATISFACTORY.