After yesterday's announcement of Android Oreo, you can now dowload it if you have one of the following compatible devices: Nexus 5X, Nexus 6P, Nexus Player, Pixel C, Pixel or Pixel XL. If you have already installed Developer Preview 4, an update to Android Oreo will be suggested to you, otherwise you will have to install Oreo yourself. In order to do this you should back up any data on your devices and then download the factory images.
To help you through the process, you can follow this handy guide.
Here's a non-exhaustive list of the smartphones and tablets who will receive the update to Android O: