Assuming you're still lucky enough to have an O-ring on your SPAS 12 you may have removed it when taking your SPAS apart and not know where to put it back.  Or you simply may not have an o-ring on your SPAS at all and need to know where it goes.   Check out the following picture:

Where do Orings Go?

Set the o-ring directly in front of the piston on the magazine tube.  Position isn't terribly important as the piston will slide it perfectly in to place.  Having an o-ring will ensure your gas system is properly sealed and aid your SPAS in cycling in semi-auto mode.  Your SPAS should always be shot with a good o-ring installed, and shouldn't need replaced very often if the SPAS is taken care of.

Need an o-ring?  The SPAS 12 Project has always, and will always, provide them for free to owners that need them.