Nomad case for Pixel 3
Strangely, Oreo’s stability has not been great following the final OTA despite being pretty solid through the developer previews. Regardless, this issue seems fairly widespread (via Android Police), and I myself even noticed it quickly following the original OTA.
The problem here lies in the complete unpredictability of the reboots. In some cases, a simple task like sending a message or opening an app can trigger a reboot, but the cause is completely unknown. Some users report that Oreo’s picture-in-picture feature can trigger this problem, but it’s not the only cause.
A clear fix isn’t available yet either. Many hoped the September OTA would solve the problem, but it doesn’t seem to have done so. In my case, a full factory reset of the phone and a manual flash of Oreo solved the problem, but others say that a full downgrade to Nougat is required. Hopefully, Google is working on a proper fix for this in another Oreo OTA.