We liked the Mount Nelson - haven't stayed at O&O.
With Mount Nelson there are four or so buildings, each a bit different and all historic - also some bungalows. Our second floor room was a walk up the stairs - no elevator - not a problem, but just to know spend time choosing your room type. We had a nice view of Table Mountain. I understand its regular clientele that has been coming at that's been coming at the same time for decades reserve the same specific rooms each year. It's on beautiful grounds but we had to taxi everywhere and I'm not sure of the area around the hotel - it seems residential on onside and a rundown business area on the other.
Service at Mount Nelson was very good, and it felt like stepping back in time. There are some models of old ships as it was at one time owned by a line that sailed from the UK to Australia.