The first rollout in the U.S. happened back in mid-March, with Verizon Wireless surprisingly leading the way. This means that U.S. Cellular is rolling out the Galaxy S8 Oreo update about two months later, but that shouldn’t be an issue now that the OS is here.
Those on the Galaxy S8 are receiving an update with baseband version G950USQU3CRD4 while users of the Galaxy S8+ are getting baseband version G955USQU3CRD4. The updates are rolling out over the air and as such, you’ll need to be a little patient before the OTA notification arrives.
If waiting isn’t part of your DNA, you can as well try to force the update via Settings > About device > Software update and download the update manually.