Actually, mon frère, Apple began using Mac OS at the release of Mac OS 7.6.I know I'm being pedantic, but the name was System 8. Apple didn't use the Mac OS name until pretty recently.

System 7.5 and Mac OS 7.6: The Beginning and End of an Era - Low End Mac
System 7.5 and Mac OS 7.6 introduced many new features and greater modernity while staying within reach of most early Macintosh models. I’m happy to report that System 7.5 and Mac OS 7.6 are the real deal. These versions of the Mac OS make connecting to and communicating with modern computers...


System 7 - Wikipedia
Good news - you would have needed to be correct to be pedantic! 😜