High Quality Content by WIKIPEDIA articles! OpenStep was an object-oriented application programming interface (API) specification for an object-oriented operating system that used a non-NeXTSTEP operating system as its core, principally developed by NeXT with Sun Microsystems. OPENSTEP (all capitalized) was a specific implementation of the OpenStep API developed by NeXT. While originally built on a Mach-based Unix (such as the core of NeXTSTEP), versions of OpenStep were available for Solaris and Windows NT as well. The software libraries that shipped with OPENSTEP are a superset of the original OpenStep specification.