They use it like OSX and iOS which rightly have OS in their name: both are the new and original operating systems. Debian, Ubuntu rightly don't have OS in their own names. Elementary shouldn't too.
1. "Ubuntu is the world’s favourite free operating system" - ubuntu.com
2. "Debian is a free operating system (OS) for your computer. An operating system is the set of basic programs and utilities that make your computer run." - debian.org
Then you have CentOS and their ilk. But that whole argument is irrelevant: take a step back and look at what you're arguing about. It is such a petty argument. I feel like I'm living in a crazy house where trivial things are important.