“This is the first step in what will become our own custom ROM. It’s so early, in fact, that this build contains no extras beyond the stock features of AOSP Lollipop. It’s the base that we will continue to build upon, improve, and customize as we stabilize core functionalities on the latest firmware updates,” says the post.
OnePlus has also enlisted certain known issues that it is working on. For instance, data roaming will be on be default, camera and video may be unstable, 4K video is supported in the alpha build but quality is not up to the mark, clock crashes periodically, Wi-fi might be unstable, on first boot start time might be slow and so on.
This build is ideally being released so that OnePlus is able to identify issues cropping up via feedback from users and work on them to come up with a stable ROM.
The instructions to flash your OnePlus One and install the new ROM have been given step-by-step on the OnePlus forum.
https://forums.oneplus.net/threads/rom-official-5-0-android-lollipop-alpha.223252/
