OS X

Ellipsis of Mac OS X; from OS (“operating system”) + X (“ten”, Roman numeral), as a successor to Mac OS 9.

• (Received Pronunciation) IPA^(key): /əʊˌɛsˈtɛn/
• (General American) IPA^(key): /oʊˌɛsˈtɛn/
• (Received Pronunciation, nonstandard) IPA^(key): /əʊˌɛsˈɛks/
• (General American, nonstandard) IPA^(key): /oʊˌɛsˈɛks/

OS X

1. (computing, trademark) Former name of macOS; itself formerly known as Mac OS X.

• “What is an operating system (OS)?”, in Apple Support‎^[1], 14 November 2007, archived from the original on 26 February 2009: “The current version of Mac OS is Mac OS X (pronounced "Mac O-S ten").”

• SOX, Sox, sox