OnePlus is looking for individuals to join its Closed Beta Program, which will give them access to early OxygenOS builds and in return requires them to provide their feedback on experimental features and report bugs. Any individual who owns a OnePlus 6 device, is active on OnePlus' community platforms, and is willing to work closely with the OnePlus team to communicate issues on Slack is eligible to sign up for the program. From all the applications that the company receives, a hundred candidates will be chosen to be a part of what the company considers to be an elite crew of its community members. The participants will be given access to unreleased software and features much before they're made available publicly as final builds to help make OxygenOS more stable for the next testing stage.
Given the need for secrecy at this early stage, the selected candidates will be asked to sign a non-disclosure agreement (NDA) with OnePlus. Once selected for the program, the individuals will be issued unstable 'alpha' builds several times in a month which they'll be required to flash on their OnePlus 6 units, and in certain cases, a clean flash must be performed, resulting in data loss. If the OnePlus 6 is your primary phone then opting for the Closed Beta Program is not recommended since the builds released to its participants will most likely include major bugs that can severely hamper your user experience. Having said that, the signup process includes filling up a simple form that will help the Chinese company better understand your usage pattern and if you're fit for the task.
After the Closed Beta Testers approve an OxygenOS build, it is released to a much wider group of Open Beta Testers to iron out any minor bug or issue before it is released as an official update to the public. Now that Android 9 Pie is out, the OnePlus team must be planning to push an OxygenOS update based on the latest Android release sometime in the near future to its handful of smartphones, hence the Closed Beta Program, though no official announcement has been made yet.