Currently there are only 6 devices that can already use the beta version of Oreo, namely Nexus 5X, Nexus 6P, Pixel, Pixel XL, Pixel C and Nexus Player.
To download the beta version of Oreo, you simply visit the Android Beta Program page from Google. Then sign in to the Google account you're using on the above-mentioned device.
After that you simply press the "Enroll Device" button next to the image from your compatible device.
By pressing the button, you also agree to be eligible to join the beta test program. Then, you can download the Oreo update to your phone or tablet.
Once you have agreed, the system update to Android 8.0 Oreo will appear immediately, ready for download.
The download process was not too long. This is the easiest way to download the latest updates from Google. System Image from Android 8.0 is also available and ready to be downloaded and installed manually if you want.
Meanwhile, how to stop participating in the Android Beta Program is also quite easy.
You can simply access the same page when registering to re-use a more stable version of the OS.
Although Oreo is still in beta stage, it is stable enough so that if you decide to try to use it it will not be too risky.