This is because OnePlus has recently released the Android Q Developer Preview 5 for both phones, meaning that you can take the update for a spin to see how it will be like on your phone ahead of its release. However, given that this is the developer preview, it means that it is sort of like a beta so it might not be the best idea to use it as your daily driver.
OnePlus acknowledges as much in a post on its forums saying, “As the name suggests, this build of Android Q is best suited for developers and early adopters. As it is still in an early stage of development, we do not recommend flashing this ROM if you have little or no experience in software development or flashing custom ROMs. Proceed at your own risk!”
If you are still interested in proceeding to download the preview, head on over to the OnePlus forums where the company has provided the instructions and the files you need to install it on your phone.