The first iPhone ran iPhone OS 1.0
https://web.archive.org/web/20070112064939/http://www.apple....
You could argue that the iPhone currently still runs macOS if you used the same definition today. They share kernels (iirc Apple always kept the ARM patches to Darwin closed-source), BSD-based userlands and the iPhone used versions of the macs application libraries.
A big difference is the iOS and macOS use different compositors.