Xerox Operating System

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| name = Xerox Operating System (XOS)

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| developer = CII

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| working_state = Historic

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| supported_platforms = SDS Sigma series

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The Xerox Operating System (XOS) was an operating system for the XDS Sigma series of computers "optimized for direct replacement of IBM DOS/360 installations" and to provide real-time and timesharing support.{{cite web|url=http://www.bitsavers.org/pdf/sds/sigma/xos/XOS_Fact_Sheet_Aug72.pdf|title=XOS Fact Sheet|date=August 10, 1972}}

The system was developed, beginning in 1969, for Xerox by the French firm CII (now Bull).{{cite book|url=https://books.google.com/books?id=b5for5RdBCoC&dq=xds+xos&pg=PA29|last=Strassmann|first=Paul A.|year=2008|title=The Computers Nobody Wanted: My Years with Xerox|publisher=Information Economics Press|isbn=9781427632708}}

XOS was more successful in Europe than in the US, but was unable to compete with IBM. By 1972 there were 35 XOS installations in Europe, compared to 2 in the US.