Everyone with a Pixel or modern Nexus will get their opportunity to use Android O soon.
With Android O entering Developer Preview status, this is our first opportunity to download and run a preview version of the latest OS. Unfortunately for those who became accustomed to simply enrolling in the Android Beta Program and getting builds over-the-air, this first Developer Preview of Android O isn't available in the Beta Program.
If you want Android O right now, your only way to get it will be through manually flashing an image of the Developer Preview.
And that is a great indication of who should actually be using the first publicly-available build of Android O: developers who need the system image to start working on updating their apps for the new platform version. When Google feels it's stable enough to push out to Beta Program users, which tend to be more of the realm of enthusiasts eager to see the new interface and features, an update will arrive to everyone who opts into the Android Beta Program.
Most of us should just be patient and wait for the Android Beta Program update.
Considering that Google just released an updated version of Android 7.1.2 to the Beta Program, this timeline of waiting until after the first Developer Preview makes sense — even though the first Developer Preview of Android N did land on the Android Beta Program the same day as the flashable image last year.
Google's timeline for the Android O Developer Preview lists mid-May for the Dev Preview 2, and specifically calls it a "beta" launch to the Dev Preview 1's "alpha" state. There's a great chance this means that the Android Beta Program will get in on the Android O fun with this release. It is set to coincide with Google I/O 2017, naturally.
But we don't know for sure yet. We'll just have to wait and see. For now, if you want to try your hand at Android O on a non-critical device, you'll need to manually flash the system image yourself.
For the vast majority of us who will plan to wait for the Android Beta Program release, expectations are that it will be available for the Pixel, Pixel XL, Nexus 6P, Nexus 5X, Pixel C and Nexus Player once released.