Some insight to the Airbus O2 sys,
The HP pressure regulator transmitter is directly linked to the O2 bottle, this transmitter send the high pressure indication to the ECAM door/oxy page. when the O2 crew supply PB on the panel (211vu) is pushed to the off position after an inbound, it shows on the ECAM crew pressure in Amber (REGUL LO PR), IF for argument sake this was missed by the flight crew during the check list prior to departure, the Amber MSG of the crew O2 pressure QTY indication on the ECAM will always be displayed in amber. When the flight crew enable the O2 crew supply PB switch, this will allow the electrical valve to open and allow O2 distribution to the system and the MSG will be displayed in Green assuming the O2 bottle Manual shut off valve is OPEN. However all fail, with the bottle wire locked in the CLOSED position and having the PB switch enabled, the low pressure switch in the supply system will be activated (LP pressure less than 50 psi) and display a MSG on the upper ECAM.I think you all familiar with the MSG. ECAM HP pressure will indicate Zero psi when the valve is closed!!.
It seems to me that when the bottle was replaced and possibly the engineer had too many aircraft to deal with on his own, He may have just forgot to do the functional check and he may forgot to mention (ops check c/o) following the O2 bottle replacement in the tech log. No to mentioned the crew forgot to look at the DOOR/OXY page upper right hand corner.This by the way is not uncommon problem.