Android O lets developers adjust icon shapes so that they're able to modify the icon to fit a unique user interface solution. While Android Nougat with the Pixel launcher has introduced the use of round icons for most of Google's own apps, and a template for developers to work off so they can make round icons of their own if they choose, Android O is sidestepping this a bit and allowing developers to utilize the new Adaptive Launcher Icons tool to modify their icons for various OEM devices.
Google explains that device OEMs can provide an icon mask that allows each icon to render in the same shape so that things look unified across the system including on the home screen, in shortcuts, in the settings app, in the sharing dialogs, and on the overview screen. App launcher icons in Android O will consist of two layers, the background and foreground layers that developers will need to use, and then for any icons that aren't the same shape a device OEM's chosen icon mask can take over and transition those icons into whatever shape was selected, whether that be a circle or even what Google is calling a Squircle, which is a blend of the circle and square shape, or simply a square with more prominently defined rounded corners.
While unification is the main idea of Adaptive Icons, Google has also made it as easy as possible for developers to fit their icons into this new system so they don't have to create a completely new icon to match the rest. This is all thanks to the icon mask which provides the overall shape. This might be new to stock Android, but icon masks have been supported on Android for years with the pairing of a third-party home launcher and the numerous icon packs that you can find on the Play Store. The nice thing now is that developers, and in turn users, won't have to reply on these icon packs to make things look unified, Google is giving developers and device OEMs the tools to make this all possible through the standard launchers. That being said, this is just the first developer preview and things make change with later previews as well as with the final version of the software when it goes public.