From everything I've read the Opus 7's are a super aggressive boot. I tried on a few Celsius boots (Climate, Cirrus, and CLS) at my mtn side shop and they fit pretty true to size. I'm a 10.5 on the scale and a 10.5 is what fit the best. I have M/L Force and the Cirrus fit into them no problem. They actually have a smaller outer profile than my current size 10 vans. I would assume the Opus sizing is similar to the others.
I picked up a pair of Cirrus and I'm pretty happy with them so far. They're much lighter than my Vans Andreas Wiig's and the OZone lacing is solid. The new liner lock is the best I've ever seen, it gets so tight that it hurts. Plus it stays put all day. The only knock on them is the toe box is large and I had some up and down play with my toes. I put in 1/8 inch boot shims on top of the liner and it solved the problem. I'll be recommending these for sure.