My wife uses this rig. It's her first back inflate BC, and she's wrapped with it. She was worried the lack of a chest strap would affect the stability, but she reported it was fine. No STA is required, however the soft STA is an option if you feel you need it. At 5'/150cm she's pushing the boundaries of the fit, but it's more comfortable for her than a regular backplate.
There is no backplate for the O-Pac - it's a soft pack, and has no provision to accept a backplate. You just buy the O-Pac, add a wing, and it's ready to dive once adjusted. It comes with camstraps, and some removable padding - she dives hers without the lumbar padding. The wing is attached by threading the camstraps through the wing and softpack, and adding a couple of heavy duty zipties (mouthpiece zipties work well) through the pairs of grommets in both the O-Pac and the wing to keep it all together. All of the load of the tank's weight is borne by the softpack, so there's no load on the zipties. Works surprisingly well. The harness is Hogarthian.