The Android O developer preview is available for the Pixel, Pixel XL, Nexus 6P, Nexus 5X, Pixel C and Nexus Player.
Remember, that doesn't mean Android O will be released for all of those devices, as we saw the Nexus 5 get developer previews for Android 7 and it was not part of the release. There is a lot of work behind the scenes with licensing and software versioning so just because a device can run the software doesn't mean it will officially see it.
I can almost say for certain that it will not come to the N6 officially...
Actually, that kinda is why I'm keeping my N6, but not because I expect to get an OTA update served up by Google. I gave up on those last November.Just read somewhere that the nexus 6 is getting android O. Will you keep the phone till then?