Above - The innards of a PLSS
Center photo has interior details labelled

Above - NASA PLSS Diagrams
Left - Note the location of the LiOH canister, and stowed antenna on top of OPS
Left Center - Location of fill and drain ports
Right Center - Excellent diagram showing location of indicators, dials, and controls
Right - Ther interconnections between the PLSS, OPS, and RCU

Interesting photo of Armstrong practicing recharging the water system

The OPS was designed as an emergency backup system to provide oxygen to a crewman while on EVA on the moon in the event of PLSS failure.  In combination with the Buddy Life Support System (BLSS) for cooling, it could give the crewman enough life support time to return to the LM and repressurize the ascent stage cabin.  It was primarily made up of a pair of oxygen cylinders and an actuator cable which was mounted on the side of the RCU.
To run the system, the crewman pulled down on the actuator cable to start flow of O2 then opened the diverter valve of the front of the suit to create a presure gradient which would force the O2 into the suit.
The OPS was covered by a plastic shell which in turn was covered by a Beta cloth cover.  The American flag was stitched to the back of it.
The OPS was carried on board the LM into lunar orbit to serve as a backup system in case the LM crew had to make an emergency EVA from the LM to the CSM.  On the later J-Series missions it was also used by the CMP and LMP during the SIM Bay data retrieval EVA.  None of the OPS flown on Apollo were ever used in an emergency situation, a great testimonial to the reliability of the PLSS.

Left - Diagram of the RCU control displays, switches, etc.
Center - Photo of top of RCU and electrical connector to the PLSS, with labels
Right - Similar to center photo