While the marketing name of Android O is still not known, we do at least know the version number. In Developer Preview 3, which was released yesterday, the version number is clearly listed as 8.0.0 in the System>About page. While there wasn’t a huge amount of mystery around this, it does finalize it.
The general assumption was that O would be v8 but the v4 train of Android (Ice Cream Sandwich, Jelly Bean and KitKat) created the confusion. All of these were v4 but were 4.0, 4.1, 4.2 and 4.3. Since then, there has always been a lingering thought that the next major version of Android could remain in the Nougat v7 lineup.
To be fair, it was always unlikely that Android O would be v7.x. Google has made it pretty clear starting with Nougat that .1 and .2
update would remain in the train as minor updates while major version would get a major version number update. So far they have stuck with that versioning and with DP3 showing the version as 8.0.0, that leaves room for minor 8.1 and so on updates too.
Probably the only remaining question will be if the 8.0.0 build is what actually ships or will it be an 8.0.1 or even an 8.1 by the time we get to the Q3 release.