Feedback from early adopters suggests that OxygenOS is close to vanilla Lollipop with a few customisations but not nearly as many as Cyanogen offers. Nexus meets OnePlus, as it were. A few known issues are present and installing the drop isn’t for the faint-hearted as the deployment is not an OTA update; flashing the ROM is required. OnePlus have produced a short video showing off the OS’ capabilities.
https://youtu.be/Tn4hHhIHLxk
Early adopters have reported a few issues too, which leaves me in a quandary as a OnePlus One owner. I’m a big Nexus fan having owned the Nexus 4, 7, 9 and 10, and I think OxygenOS will suit me well. However, I moved away from the Nexus 4 after the first Lollipop release ruined the phone (sometimes calls connect but the caller can’t be heard) so I can’t afford another half-working phone. At this point, I’m going to hold off until there have been a few updates but I’m looking forward to a long-term future with OxygenOS.