What was found
(CAN) DURING SCHEDULED ACFT MX, A REPLACEMENT OXYGEN BOTTLE WAS INSTALLED. FLIGHT CREW LATER DISCOVERED THE OXYGEN SYS WAS NOT WORKING PROPERLY. THE OXYGEN SHUTOFF VALVE HAD BEEN RIGGED BACKWARDS TO THAT OF NORMAL OPERATIONS, MEANING WHEN THE COCKPIT HANDLE WAS MOVED TO THE SYS READY PULL ON POSITION THE OXYGEN VALVE AT THE BOTTLE WAS SHUTOFF. THE VALVE LEVER IS ABLE TO ROTATE 180 DEGREES AND HAS ON/OFF MARKINGS ON ONE SIDE ONLY. POST INSTALLATION CHECKS CARRIED OUT CONSISTED OF LISTENING FOR THE AUDIBLE FLOW OF AIR WHEN ARMING THE SYS. IN THIS CASE THE AIR FLOW HEARD WAS THAT OF DEPRESSURIZING THE SYS. OXYGEN VALVE RE-RIGGED CORRECTLY AND PROPER PRESSURE AND FLOW OBSERVED AT CREW MASKS.