Mac OS X Server 1.0, released on March 16, 1999, is the first operating system released into the retail market by Apple Computer based on their acquisition of NeXT. It is a version of Rhapsody series of developer releases of what was to be known as Classic Mac OS X. Although OS X Server 1.0 had a variation of the Platinum "look and feel" from Mac OS 9, it is based on the OPENSTEP (and thus, NeXTSTEP) operating system instead of the classic Mac OS, giving users a preview of the future operating system Mac OS X. It was replaced by a version based on Mac OS X v10.0 in 2001.
Read more about Mac OS X Server 1.0: Design, Release History