The first customers to receive the update are testers that have been in Samsung’s beta program for the past few months. However, according to SamMobile, everyone else currently running Android 7.x Nougat will start receiving the update starting tomorrow (at least in the German market). Additional rollout timetables will depend on what region you live in.
Rather amusingly, the firmware version is G955XXU1CRAP, although we're glad to say that Android Oreo is far from being crap.
Also along for the ride is the Samsung Experience 9.0, which includes Samsung-specific UI and UX enhancements for the operating system. And besides all of Samsung's special sauce that you'll find sprinkled throughout, you of course get all of the yummy goodness that comes bundled in with Android Oreo.
Android Oreo can now boot up to 2x faster than its predecessor, limits can be placed on background app activity to enhance battery life, picture-in-picture support is added (helpful if you'd like to watch a video while browsing the web), along with enhancements to Autofill and Smart text selection. There are also tons of minor updates strewn throughout the operating system and a number API changes (and additions) under the hood.
If you want to see if your Galaxy S8 or Galaxy S8+ is ready to accept Android 8.0 Oreo and Samsung Experience 9.0, navigate to Settings --> Software Update on your smartphone. If you don't see the update available right now, don't be alarmed -- it will probably come alive within the next few days.