OnePlus offers in-depth details on OxygenOS beta program
Basically, these community builds for OnePlus' OxygenOS are parts of the company's new, expanded software beta program. Their purpose is to involve the community in the development of OxygenOS. Also, this is a good way to test every feature, tweak, and addition to the OxygenOS.
The open beta is available to all OnePlus users who want to install OxygenOS test builds on their phones. As the name suggests, the official release is meant for all compatible OnePlus smartphones and are supposed to be bug-free.
OnePlus announced that it will provide around 4 OxygenOS community build updates for closed beta per month, 2 builds per month for open beta, and 1 update every 1-2 months for the official release “stream.”
All closed beta releases are referred to as “alpha builds” since they aren't stable at all and they lack the polish of an official release. OnePlus will deliver more OxygenOS updates for the closed beta “stream” since new features are added all the time.
The open beta builds are all about fine-tuning and make the software as stable as possible by fixing and bugs found during tests. These are usually semi-public, which means that everyone can try them out.
source: OnePlus
Things that are NOT allowed: