I think it's matter of perspective, and also depends on the ship.
We sailed on the Riviera in May and on SS last December, and we found both lines comparable in terms of food and service. What makes SS more "luxury" than O is the fact that they are all inclusive, plus larger cabins and better space/passenger ratios. All other aspects - I didn't see anything on SS that makes it more luxury than O.
On related note, we would sail on both O class and R class, but still consider the O class more "luxury". Much better cabins, newer ships, more ding options etc. I think that PH on the R class is not a good value (just my personal opinion).