If you want to avoid a long download process when Android Oreo arrives, make sure you keep your Galaxy S8 up-to-date with the latest software.
Samsung's updates are sporadic, but it's still easy to fall behind. The latest updates for the Galaxy S8 deliver bug fixes and security patches and most of you should download the latest version for your device.
If you aren't sure what version of Android you're currently running, go into your Galaxy S8's Settings and head into the About section to check your current version.
If your security is outdated or you haven't updated in awhile, head into your Galaxy S8's settings and check for a software update.
Upgrading to the latest version will protect your phone and it will save you some time when Android Oreo prompts on your device.