Android O is currently in beta and Google were taking requests from users to test it, unfortunately they are no longer taking on testers however, there is another way, first backup your device then inform your device that you are a developer even if you are not.
Navigate to the “about phone” section and scroll down to where the “build number” is listed. Click this box seven times and you will be a “registered developer”. A new option called “developer options” pops up on the previous menu.
Simply click developer options, Make sure that both “OEM unlocking” and “USB debugging” are both set to the “on” position as this will allow both the bootloader to be unlocked with a simple command (detailed below) and for your computer to be able to send files to your device.
Once you’re all set, we’re ready to download the system image from here. Download 15 seconds ADB installer from here, This application installs all of the necessary files that will allow your PC to communicate with your phone and send over the new Android O software.
A window will appear asking if you want to install ADB and Fastboot. Type Y on the keyboard to agree. Press Y again when asked to Install ADB system-wide and if you want to install device drivers. A new window will open to start installing the drivers. Allow this, then click restart now to reboot your PC. The program will now be installed to file adb.
Plug your phone into your computer, download the Android O image for your device, you’ll need to shift it around to a different location on your hard drive. Right click it and unzip the .zip file using a program like 7Zip and set it to extract in C:/adb, if that’s your default drive letter. Power down your phone and ensure that it’s plugged into your computer.
Now, you need to start it in fastboot mode – to do this on the Nexus 6P, press and hold down the Volume Down and Power buttons on the handset from the off state. Same goes for the Nexus 5X, Google Pixel, Google Pixel XL and Google Pixel C.
Once your device has booted in fastboot mode, open up the folder on your PC where you extracted the Android O image to. Click into the folder until you find a file called flash-all.bat. Double click this to begin installing the new software onto your device. (Refer to the image above if you’re confused at this point)
The next and final step: be patient! If all goes well, the process should take about five minutes. Unfortunately, there’s no progress bar, so you’ll just have to trust that things are going well. You can watch the process unfold in the command prompt menu.
Original source techradar