As OW Sapphire on a BA metal HBO fare:

You WILL get:
> Lounge access at LGW/LHR (no lounges at LCY, if your flight goes from there)
> Priority boarding at LGW/LHR, for what it is worth these days since half the flight has it. No priority boarding at LCY, that is for BA Gold/OWE or business class pax only.
> Free seat selection if you book before 26 Mar 2015. After this, nobody gets free seat selection on an HBO fare, even BA's own OWE (the cause of much anguish currently on the BA forum)

You will NOT get:
> Any free checked bag. That applies to everyone, including BA's own status pax. As JAXBA says, it is definitely cheaper to buy this when making the original booking, but if not, it will still be cheaper to buy online before getting to the airport, than it will be at the airport, should you need to check a bag.

You MIGHT get:
> Fast track security at LGW/LHR - it's a bit hit and miss, technically it's only for BA status pax if travelling in Y (and even they sometimes get questioned or refused), but depends on the agent checking the boarding passes at the entrance. Give it a try, as long as you don't mind if they turn you away. There is no fast track at LCY.

If there is possibly some confusion about your status registering across the carriers in bookings, recommend that you carry your physical card showing your OWS status for the lounge (and boarding, though technically you should still have it as OW Ruby).