I evenly use both. So it doesn't bother me. I use o (oh) for example if its in the first 3 numbers of a phone number. But I say 0 (zero) if its in the last 4 numbers.
Originally Posted by The Dude
"Let me explain something to you. Um, I am not "Mr. Lebowski". You're Mr. Lebowski. I'm the Dude. So that's what you call me. You know, that or, uh, His Dudeness, or uh, Duder, or El Duderino if you're not into the whole brevity thing."
"O" (oh) is slang often used in the USA. It is not a standard word of the English language as far as I can tell. The reasons why to say "Oh" instead of null, nil, zero have been mentioned: faster, shorter, personal preference, cultural difference