Not all devices will run Android Oreo. Those that do will receive update notifications gradually over a period of time, so that there aren't millions of devices all trying to download it simultaneously.
However you can check whether there's an update available for your device simply by going to Settings > About Phone > Check for update.
The development of Android is at a rather odd stage as Oreo (Android 8) is only just starting to be rolled out for most phones but Pie (Android 9) is already available in some new phones (such as the Sony Xperia XZ3).