Air carrier Captain reported there were oxygen bottles that had been depleted and needed replacement that caused delays to the flight. There were also conflicting statements on the write-up requirement and overall; the correct procedures for this issue.

Narrative

When cockpit door was opened from previous flight a man walked into the cockpit and opened up the spare bulb door and started looking through it. I asked him who he was and why he was going through the cabinet. He then explained he was Maintenance and he was looking for the O2 adapter. I then started to help him look for it because he explained that the ZZZ to ZZZ1 flight was down due to 2 O2 bottles being depleted. Once we could not find the adapter I called Dispatch and transferred to Maintenance Control. I specifically asked if I needed to write up the fact that we did not have the O2 adapter onboard and if we could give the other aircraft 2 bottles to get them on their way.I was told I do not need to write up the adapter and they were trying to fill the bottles. I now knew this was going to somehow turn into a delay for me because neither aircraft had the O2 fill bottles.Here is the concern. I once had the O2 fill adapter on Non-Essential for Flight (NEF). That NEF was 25-XX; the flight suitcase. I emailed the previous Airbus Fleet Manager where I could find a list of the flight suitcase. I never heard anything from him. So I have two conflicting statements. One saying it has to be on NEF and now one stating it doesn't. When I landed in ZZZ2 the Mechanic said a logbook page had to be filled out putting the adapter on the aircraft. So I do not know what has to be done here. The Mechanic in ZZZ also stated that the O2 bottles that we swapped had to be removed from the aircraft due to them being hazmat. The MEL states that if" they are removed they are considered hazmat. So now does the aircraft Basic Operating Weight (BOW) have to be amended because the bottles were removed? All this caused our passengers and subsequent flights to be late.I tried to get ahead of this. Instead of sending a Mechanic to look for a fill adapter in places it could possibly be; i.e.; the Avionics Bay; the stowaway kit; just swapped the bottles like I suggested. I was told they didn't want to do this because it would have caused a lot of paperwork. Which we ended up doing anyway. Also have a designated place for fill adapter or procedures when it's not on board."