The dedicated Android Beta Program site seems to have undergone a change fairly recently. Not one which has any immediate benefits to the general public, but one which is an informative change. This is the page that is setup to allow users to register their interest (and/or their devices in the Android Beta Program), and which up until now has been reserved for Android 7.0 (Nougat). However, this is where the change is - as now the page seems focused on Android O.
Android O is not yet ready to be distributed in a proper beta version, however, the website does seem to have been updated in preparation for when the beta version becomes available. Of course, Android O is already available in some capacity, although, this is more thought of as an alpha release of Android O. After all, it is not only an early-access developer version, but it is the first developer version to be released. One which Google itself refers to as an alpha and initial release version. Therefore, the beta release of an Android version is often a slightly more consumer-ready version, and one which will run and be used for testing right up until the final release.
Which brings us to the main point here, while Android O is not yet ready in true beta form, the website does make it clear that the Android O Beta is coming soon. How soon though remains less clear. This month sees Google I/O taking place where it is expected that more news on Android O will be on offer. Likewise, the next developer previews (DP2) is expected to come through around I/O, or shortly before. Which is likely to be the start of the beta process for Android O. Which is also therefore likely to be when the Android O Beta Program will officially open for business. As per usual though, you will need to have a relevant device to be able to take part in the program, which typically speaking, means a Nexus or a Pixel device. And of course, sign up to be part of the Android O Beta Program, when it does finally open.