Neevo audiotasks pronounciation of 'O'. Is this pronounced as 'oh' or 'zero'.

Specifically in relation to a line of numbers, usually part of addresses, and written such as; 'Five four zero one O three'.

I've heard it pronounced both ways and now I'm doubting myself as to which one is right, especially if there's already a 'zero' written in word form in the same line.

Thanks for any help (it's not in any of the specific guidance docs I've read).