Of the two sets of devices, only the Galaxy S8 family is receiving an Oreo-based update while the S7 trio is still on Nougat. The updates install the same security patches, though, for the month of April 2018.
Where the S7 trio is receiving an update with build number BRD1, the Galaxy S8 and S8 Plus are on build number CRD4. As for the Galaxy S8 Active, the update carries build number BRD4.
All six phones are receiving the update over the air and in fact, AT&T says it started rolling out the updates to five of the phones on May 9, with the S8 Active getting it earlier on May 7.
If you have not seen the download notification, don’t fret. OTA updates may take days or even weeks before everyone gets them. Alternatively, you can try forcing the update via the device’s Settings > About phone > Software update and download the update manually.