People write their own software from scratch all the time. People don't write their own OSs all the time because there's no point, and it would take far too long for 1 person to do it.
Making an operating system on your own is... a complete waste of time to be brutally honest.
From device management to disk management to error handling to ... to... to... to everything... its simply nuts.
That said, i did code a lot of the stuff i have on this OS (Linux). that is probably the closest thing of "I did my own OS" you will hear from single person.
I've worked on pretty much all aspects of Operating Systems. I've written boot-loaders, device drivers, low level graphics hardware access (back in the Dos days...). You name it. I've been wanting to write a toy OS in Haskell for a couple years now, but have never gotten around to it. There are a bunch of toy OSes around the net that you can check out, as well as many tutorials on rolling your own OS.