I'd also like to add that the standard Rotor Q ring can be persuaded to go onto a Shimano Ultegra compact 110 bcd 2011/12/13 crank. The outer ring initially won't fit onto the crank as it won't go past the oversize spiders that Shimano puts on to make the chainring look pretty with their proprietary chainrings but, with a little bit of grinding they will go on then you get all 5 OCP options.
Essentially you'll need a Dremel or similar and about ten minutes. You have to grind away a little of the ring around OCP positions 2 & 4 and when you offer up the ring to go on you'll see it doesn't want to pass over the spiders at the 3.30 & 8.30 o'clock positions, if the crank arm is 12 o'clock (difficult to describe without a drawing!). With some work you can cut two little notches in this position and after a while you'll find it then slides over the spiders. It'll then get stuck on the one at the 6 o'clock position but, if you put the whole lot in a vice and give it a squeeze, it'll pass over. To make the whole lot easier I ground small 'ramps' onto the crank spiders so I didn't have to take as much out of the ring and this helped it to slide over but some may not want to do this as they don't want to mark their original crank. As mine was the charcoal version I used a marker pen to hide the metal colour on the parts I'd ground and the whole lot now works a treat..!
I really don't know why Rotor don't do this themselves as it would save them making two different rings (the OCP 3 and the Standard one) and it'd be a lot less work with their CNC milling machine than it was for me with my ten-quid Dremel and some grinding bits... The metal removed is mere millimetres and has no effect at all on the function or stiffness of the ring.
As the poster above implies, the standard ring is much stiffer than the OCP 3 one and you get all OCP positions although, in honesty, I can't tell the difference between any of them so the point is moot as to whether this really matters.
Finally I'll add my two-penn'orth to the debate as to whether they actually do anything. I can tell no difference at all pedalling or, if I can, it's as likely to be my brain persuading me my 160 euros was well spent as to any real difference in feel or efficiency. I don't climb any quicker nor go faster on my rides. Everything is exactly the same but 'perhaps' they feel a little smoother to pedal. I'm using 50/34 rings, weigh 85kg and live in the Alpes so I climb a lot. I'm a spinner, not a grinder, as many big lads tend to be... The only benefit, and it's a massive one, is my knee has stopped hurting after 500km of riding. I'm an ex-pro snowboarder and have the usual injuries to show for it which include no ACL in my right knee and grade 4 (to the bone) cartilage damage on the back of my kneecap. I partially cycle to keep this strong and to help the arthritis but also to keep weight down. The pain is related to the missing cartilage, not any sort of muscular or tendon injury. Whatever the rings are doing is working as, like a lot of pain, I only noticed it had stopped when I said on my last ride - 'wow, my knee's not hurt me for about a week'. We can't remember pain so you're only aware it stops when one day you notice you're not feeling it any more. Anyway, they seem to have helped this out but I'll only really know if this remains the same with another 1000km or so on the rings. My knee has more-or-less hurt ever since I started cycling again 3 years ago after being a good MTB racer in my youth (I'm now 45). It's never not hurt, only varied between 'really annoying' and 'slightly annoying'. It's now 'non-detectable' but whether this is the rings or, like a lot of chronic pain it's going through a quiet phase, I can't be sure...