[Update] A post on OnePlus' forums clarifies that the OS is not open source, "OxygenOS will not be open sourced. However, we are releasing the kernel and the device tree to the community for development." Further, "That article is factually incorrect. We've contacted XDA already."
OnePlus has open-sourced its Android-fork, OxygenOS. This means that developers can download the source code and develop their own fork from it. According to a report on XDA forums, users can also try and fix issues in OxygenOS. If the fixes work, they can send a request to OnePlus and the company may choose to incorporate them into the next update. Developers are also free to build upon the features of the base code of OxygenOS and create their own custom ROM. Instructions for downloading OxygenOS can be found on GitHub.
This move comes just a few days before the launch of the OnePlus 3 and maybe OnePlus is harking back to the days of the developer friendly OnePlus One that was launched with Cyanogen OS. However, things soured between OnePlus and Cyanogen Inc. after the latter signed an exclusivity deal with Micromax. The company expressed its disappointment and started working on its own UI called OxygenOS. The new OS debuted on the company’s next flagship device, the OnePlus 2. The company is also in the process of rolling out the OxygenOS 3.0.2 update for the OnePlus 2, which brings Android Marshmallow v6.0.1 to the phone. The update brings the usual suite of Marshmallow features along with system improvements.