I’ve read the history of it, and it’s added up above in a higher level comment. Both 0 and O were used to refer to the absence of antigens, and it was an Austrian who made the final recommendation that stuck. Which is why it’s all the stranger that German-speaking countries kept the zero.
Just a wild guess by a fellow Austrian: we do not say O instead of 0 (like in phone numbers) in German. So it just makes no sense for average Joe to write an O. There is no connection here (we have the tramway line O though in Vienna).
I’ve read the history of it, and it’s added up above in a higher level comment. Both 0 and O were used to refer to the absence of antigens, and it was an Austrian who made the final recommendation that stuck. Which is why it’s all the stranger that German-speaking countries kept the zero.
Just a wild guess by a fellow Austrian: we do not say O instead of 0 (like in phone numbers) in German. So it just makes no sense for average Joe to write an O. There is no connection here (we have the tramway line O though in Vienna).