When you say it frequently enough, you'll appreciate the one less syllable over "zero."
edit: I even think it's more natural. "Zero day" are two real words that could be a sentence fragment, "oh-day" feels like a new word. Saying "that's a zero day" feels like an incorrect sentence to me (uncanny valley), whereas "that's an oh-day" only sounds weird if you don't accept "oh-day" as a word. To me, "oh-day" is similar to most slang, where it feels natural to say/hear even when you don't know what it means.
