I wanted a short domain name as I was experimenting with websites and QR codes. Most of the short .com domain names have been taken however some fairly short .org ones were still available. I registered 0z1.org was the shortest one that I could find.
I appreciated having short email addresses like a@oz1.org however the number 0 can be mistaken for the letter o.
I later also registered oz1.org when it became available and set up my websites and email so either oz1.org or 0z1.org can be used.