In those days Mac OS was called Macintosh System Software â `` which was shortened to System or SSW ( System Software ) . The term `` Mac OS '' first appeared in the splash screen for System 7.5.1 , but was not adopted as the new name until the release of Mac OS 7.6 . System 7.5.2 was the first to include the Mac OS logo ( a variation on the original Happy Mac startup icon ) , and Mac OS 7.6 was the first to be named `` Mac OS '' .