You might have heard OnePlus has formed a dev team to build its OxygenOS ROM. They even did an AMA on the /r/Android subreddit today. Among the bits of info revealed by the team is the fact that OxygenOS won't be open source. Instead it will be a "real OS." Steve "Cyanogen" Kondik couldn't resist poking fun at that.
Cyanogen OS as produced by Cyanogen Inc. is not completely open source either, but it sounds like OnePlus is going for a more closed approach akin to HTC Sense or TouchWiz. There are, of course, some open source requirements for anything based on Android, but it's not unheard of for an OEM to keep all the development and features closed. It's actually the default.
Look at it this way—Samsung doesn't have a git for TouchWiz where you can access the code, but it releases kernel source for each build. Apparently OnePlus will be doing something similar. Kondik points out the community development is a boon to CM, and he certainly has a point. He also says CM has "no plans of taking our toys and going home." Burn. It will be interesting to see how well the small OnePlus ROM team can operate without help from a legion of hobbyist devs.