Google released Android O developer preview images back in March. Today during Google I/O 2017 it has started registrations for the upcoming Android O beta. When it released Android O developer preview, Google said that Android O will bring new power and performance optimizations and many new ways to extend your apps.
Android O brings several new features such as notification channels, PIP, adaptive icons, along with some of the new APIs — autofill, font resources in XML, autosizing TextView, WebView enhancements, and more.
Once enrolled, you will get OTA updates for beta releases, same as Android N Beta Program. It is available for Pixel, Pixel XL, Nexus 5X, Nexus 6P, Nexus Player and Pixel C. Google is not expected to add other older devices.
If you were previously enrolled in the Android N Beta Program, you must enroll again to receive Android O, said Google. You can head to Android beta registration page to sign in using your email to enroll into Android O beta.
Update: Android O developer preview 2 images are now available from Android developer website here you can also get OTA images from here. Stable release will be available in Q3 2017.