ASUS isn't always the best with updates. The company often releases phones with already-outdated software, like the ZenFone 3 Zoom that came with Marshmallow. In a similar vein, the ZenWatch 2 and 3 were some of the last watches to get Android Wear 2.0. But at the ZenFone 4 launch event, ASUS promised that the entire ZenFone 3 and 4 lineup would get Android O.
The company announced that ZenUI 4.0 will be used on top of Android O, which is already present on the ZenFone 4 lineup. The 35 pre-installed apps on ZenUI 3.5 were trimmed down to just 13 on 4.0. It looks like ASUS is following the general trend of lighter Android skins, like HTC has done over the past few years.
ASUS has released a few phones outside the ZenFone branding recently, such as the ZenFone AR, but it's unclear if any of those devices will receive Android O. The company expects that all ZenFone 3 and 4 devices will be updated to Android O by the second half of 2018.