I located where my A/C system is loosing 134A; it is at the lower connection on the condenser.  My guess and hope is that a new O ring will fix the leak. My question..........since I will have the system open, is it a good preventive maintenance to replace the O rings on both sides of the compressor? They don't leak now, but part of me also thinks " it it ain't broke, then don't fix it.

 

Any advise??

yes! any time the system is open always replace the orings on the compressor.  those two are the most common failure points i've seen in Subaru's.  i guess heat cycling/vibrations/etc but guessing aside - it's by far the most common Subaru A/C repair I've done.  it's basically the only one other than compressors damaged components from impact (wreck/debris).

 

replace those two orings and the schrader valves in the high and low ports, those have a rubber oring at the base prone to leaking as well.

I'd even go so far as to say replace all the AC O-rings in the engine compartment.  There aren't that many and they're all pretty easy to get to.  You can get a metric kit of the HNBR O-rings at your FLAPS.

 

If you see oil/dirt on the fitting where it was leaking, you should put some oil back in.  How much is the tricky question, depends on how much leaked out.  If you removed the compressor you can get an idea of how much oil is in the system, otherwise if it doesn't seem like a ton leaked out you could throw an ounce of PAG 100 in.