That’s certainly what Malcom Ten calls it!
Not that I have heard. . . and that is not what Apple calls it. They've specifically use OS TEN in every comment they've spoken aloud about it. Only those who don't know use the wrong pronunciation. Even the numbering system is explained that way. It followed MacOS 9.
Steve Jobs announced it as OS TEN and never ever called it OS EKS.
OS X ( /oʊ ˌɛs ˈtɛn/), formerly Mac OS X, is a series of Unix-based graphical interface operating systems developed, marketed, and sold by Apple Inc. OS X is designed to run exclusively on Macintosh computers, having been pre-loaded on all Macs since 2002. OS X, whose X is the Roman numeral for 10 and is a prominent part of its brand identity, is built on technologies developed at NeXT between the second half of the 1980s and Apple's purchase of the company in late 1996. It was the successor to Mac OS 9, released in 1999, the final release of the "classic" Mac OS, which had been Apple's primary operating system since 1984. Apple also uses 'X' in 'OS X' to emphasize the relatedness between OS X and UNIX. Definition of OS X from FreeBase
You can also just ask your Mac. Open the Terminal App and type "Say OS X" and it will respond: "OS TEN". That's pretty definitive.