Jeremy was very wise to point out that in some English speaking countries, and certainly in the United States, we do add "o" or "oh" to years like 1901, 1903, 1907, and so forth.
It is obviously an error to confused the Letter of the Alphabet with Zero, but we do it anyway.
Clearly, it is easier to say "o" than to say ZERO. It is wrong, but we just adopt the simpler expression. This may puzzle some students of English to know end. They may well ask "why?". It is now custom in the USA.