Google just took the wraps off Android 8.0 Oreo, which we've known only as Android O for the last few months. The source code is being pushed to AOSP today, but OTA updates are still in process. The system images are live right this minute, though. If you've got a compatible device and an unlocked bootloader, you can be on Android Oreo in just a few minutes.
The system images are available on Google's dev site for the Nexus 5X, Nexus 6P, Nexus Player, Pixel C, Pixel, and Pixel XL. Note, there are two variants of each Pixel build. Make sure you grab the one for your carrier (if it's listed). The update package includes all the necessary files, but you need to have ADB running on your computer before installing.
Place the unzipped files in the computer directory with ADB and run the batch file (with the phone plugged in, obviously). You need to have an unlocked bootloader for this to work, and all your data will be removed. You can edit the batch file (or shell file for Mac and Linux) to remove the "-w" command to keep your data. You should always make sure you're backed up before attempting an update, though.
If you don't have an unlocked bootloader, you can wait for the OTAs to roll out. That might happen sooner than you'd expect. Some people are having issues getting the system images to install at all, so maybe waiting on the OTA is better anyway.
OTA updates have started to roll out, but not particularly smoothly.
- Android Oreo Images