Samsung Galaxy S8 and S8+ on Sprint, Verizon and T-Mobile has received Android Oreo, now the Galaxy S8 and S8+ units on AT&T started to receive the update. Many users on the reddit has received the Oreo update and the update weighs around 1.55GB. However official support pages on AT&T has yet to update the Galaxy S8 and S8+ Android Oreo details.
The update brings the February security patch and Samsung latest UI Samsung Experience 9.0 along with Android Oreo features. You can manually check for Android 8.0 Oreo by heading to Settings / System update. It will take couple of days for the update to hit all the units.