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.