There's a bunch of differences...
I'd say that OneCloud is "dialtone as a service". It's the same software, but packaged in a way that it's a managed service. You trade flexibility for standard management. Everybody uses ServiceNow, there's change management when they patch stuff, etc.
It's basically their managed services layer wrapped around hosting and a reference implementation of their stuff. It's great until you absolutely need to deviate.
To be fair, the technology stack is getting more and more complicated. 10 years ago I got away with a CM terminal and hadn't really heard of SM/SMGR yet. It ain't that way anymore. So, if you want to consume the new stuff in line with a reference implementation no fuss no muss, great. But you better fit the mold. Nothing stops you from getting a very custom solution - in anybody's cloud - designed, installed, maintained, etc by Avaya according to the same managed service metrics as OneCloud ReadyNow. Getting it quick and easy is the "Ready Now" part.