Google has released the Android O Developer Preview 1 images for the public and developers to test out. The images are currently out for last generation of Nexus and Pixel devices, the Nexus 5X, 6P, Pixel, Pixel XL and Pixel C are currently supported by these released images. You will have to flash these images manually as no OTA or Beta images will be provided.
The upcoming version of Android, codenamed Android O seems to be focussing on battery life and notifications along with audio. Battery life is something that gets better on every version of Android and now with Android O, the apps won’t have as much wakelock freedom as they have at the moment. Apps will only be allowed to wake the devices up in specific slots.
Apart from that, you can expect better notifications and you will be able to choose which types of notifications you want to see. As for the audio, there’s support for low latency wireless audio on Android O. The final version of Android O will be released in Q3 of 2017 but you can expect the Preview 2 and Preview 3 in May and June respectively.