OnePlus' beta program is an interesting facet of running OxygenOS, this OEM's version of Android. It extends users the option of previewing new features of the ROM in question, provided they are willing to use a slightly different OTA channel for the same rather than that of its Official (or stable) Release.
There are 2 of these programs: the Closed Beta, which grants access to the earliest form of the OxygenOS version in question, and is often subject to an application. The Open Beta arm, on the other hand, is available to all users and contains features that may have been vetted through feedback emanating from the former.
These Closed and Open Betas have historically entailed 4 and 2 updates per month respectively, which is a potential advantage over the stable version, which is typically renewed once a month. However, OnePlus has recently amended its OxygenOS release cycle so as to cut both beta-program monthly versions in half.
The OEM has stated it has taken this decision "in order to provide a more stable and reliable experience" to beta users. However, it just may have something to do with the steadily-rising number of new devices per year that then require these programs to keep afloat in terms of OEM- and Android-specific innovations.
The OEM does currently support the 7, 7 Pro, 7T and 7T Pro in these terms. Furthermore, they are also to be joined by their 2020 successors, the 8 and 8 Pro, shortly.