This could be wrong, but from the top of my head. The oxygen generators from the MD80, are lifed items and are removed at the end. They are not return to service items and are completly replaced at the end of their life. They have inop tags tags attached and caps are attached to stop the tags being pulled. They must be stored in the HazMat (Hazardous Materials) before they are destroyed.
Thats from memory from a Video we watched about Valujet and their poor maintainace which left a jet crashing into the swamps in Florida....