A321 Flight Attendant reported 3 Crew Oxygen bottles were discovered to be empty or below minimum pressure for service. After maintenance; the aircraft departed. The Flight Attendant then found that the bottles installed were also below service limits.

Narrative

Upon arrival at aircraft; during pre-flight checks; I was Flight Attendant 2 and noticed 3 O2 bottles missing from their brackets. The Captain was notified and informed the Crew that it was already in the log book. Boarding began as usual and on time. Towards the end of boarding; I saw a maintenance person replacing the 2 missing bottles on the aft cabin bulkhead. After he finished; I looked at the bottles to make sure the tubing was attached and noticed the top bottle gauge was not full. I could not see the gauge on the bottom bottle. During the time (towards the end of boarding); I received a crew call stating we were canceled; more info coming soon. A few minutes later; we were told the flight was reinstated. There was a bit of delay during this time; so I informed Flight Attendant 1 about the O2 bottle not being full. A few minutes later; the maintenance person had come back to the aft and pulled the bottle out of its bracket; looked at the bottle; then reinstalled the bottle. We were then told of passenger manifest issues and main cabin was deplaned. During our brief cancellation; many passengers were rebooked and not on our manifest; so that needed to be cleared up before we could leave. Once inflight; I removed the questionable O2 bottle to get an accurate reading of the gauge. This is when I realized the gauge was around 1;200 psi; not the required minimum of 1;500 psi. I then removed the bottom bottle to discover it was completely empty. While doing a cabin walk through; I stopped at the 2L door and looked at the third O2 bottle; and saw it too was completely empty. I was shocked that ZZZ had put on unusable bottles. I reported this information to the Captain that the bottles needed to be replaced again in ZZZ1. We were told during the ground delay that the reason for the cancellation was because ZZZ had no O2 bottles. Then they found some; so the flight was reinstated. Once maintenance returned and checked the gauge level and left; I figured that it was at an acceptable level as they would know the required level on the gauge to be 'legal.' We have no procedures to double check what maintenance does. There are only 6 bottles of O2 walk arounds on this aircraft. With 3 unusable; this did not leave enough bottles for each flight attendant in a decompression.