Just like last year with Android N, the Android O developer preview has been released early. This developer preview is meant solely for app developers and device makers, and is arguably very buggy at this point. Android O brings with it background limits, notification channels, standard autofill APIs, picture-in-picture video, adaptive icons, as well as support for multi-display.
Google has made the Android O preview images available for compatible devices, namely, the Nexus 5X, Nexus 6P, Nexus Player, Pixel C, Pixel, and Pixel XL. For now, no betas or OTA images are available, which means users will have to flash the developer preview onto their devices.
While the above mentioned features are being rolled out with the first build, there will be many more added in the course of time, as the final release won't happen any time before Q3 2017. Google has also revealed plans for further developer preview releases. Android O Developer Preview 2 will be released in May, Developer Preview 3 will be released in June with final APIs, and Developer Preview 4 will be released in July. The final release will be in the third quarter.
For now, Android O Developer Preview 1 brings background limits, which means apps running in the background will be more strictly controlled resulting into battery life efficiency.
Notification Channels feature lets the users control app's notification categories. It lets developers give users fine-grained control over different kinds of notifications - users can block or change the behaviour of each channel individually, rather than managing all of the app's notifications together. Android O also adds new visuals and grouping to notifications that make it easier for users to see what's going on when they have an incoming message or are glancing at the notification shade.
Another big change is autofill APIs which essentially brings platform support for autofill. This means that the service will work across ecosystem by just selecting an autofill app, similar to the way they select a keyboard app. The autofill app stores and secures user data, such as addresses, user names, and even passwords. For apps that want to handle autofill, Google has also added new APIs to implement an Autofill service.
As leaked earlier, Android O also brings picture in picture video support, along with a new app overlay window for apps to use instead of system alert window, and multi-display support for launching an activity on a remote display. Support for adaptive icons that change every day, and also support for animated icons, has been added. Apart from this there are several other developer-friendly features added, and you can read about them in detail on Google's blog post