Add OpenServer and z/OS

Author: sirredbeard

README.md

@@ -91,6 +91,8 @@ For a variety of historical and legal reasons, there has also been a massive exp
 * [AIX](https://www.ibm.com/power/operating-systems/aix)® - AIX is a series of proprietary Unix operating systems developed and sold by IBM for several of its computer platforms. 
 * [HP-UX](https://www.hpe.com/us/en/servers/hp-ux.html)® - HP-UX is  Hewlett Packard Enterprise's proprietary implementation of the Unix operating system, based on UNIX System V.
 * [UnixWare](https://www.xinuos.com)® - UnixWare is a Unix operating system made by Xinuos from the assets of SCO Group.
+* [OpenServer](https://www.xinuos.com/products/openserver-6/)® - OpenServer is a Unix operating system made by Xinuos from the assets of SCO Group.
+* [z/OS](https://www.ibm.com/products/zos)® - IBM z/OS is an operating system for IBM zSystems mainframes.
 
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