Mac OS X Server 1.0 was released on March 16, 1999, and was based on a combination of NeXT technology and Mac OS 8. Although the appearance of the OS is based on the platinum appearance in Mac OS 8, the software is based on OPENSTEP (NeXTSTEP).
Interestingly, Mac OS X Server was released almost two years before the Mac OS X operating system public beta. I remember it being used in a computer lab at a middle school in the first district where I served as Tech Director. We were testing the new NetBoot server that purportedly allowed multiple computers to boot from a disk image over a local network. It did not work for our purposes.
This is the original Mac OS X Server box that sold at the time for $499.