OpenStep OpenStep that uses any modern operating system as its core, principally developed by is an open object-oriented API specification for an object-oriented operating systemNeXT. It is important to recognize that while OpenStep is an API specification, OPENSTEP (such as the core of (all capitalized) is a specific implementation of this OpenStep developed by NeXT. While originally built on a Mach-based UnixNeXTSTEP), versions of OPENSTEP were available for Solaris and Windows NT as well. Furthermore the OPENSTEP libraries (the libraries that shipped with the OPENSTEP operating system) are in fact a superset of the original OpenStep specification.