O(1) or 0(n)

Hi I had a interview and there was a time complexity question about my code.

Basically the function was iterating through a fixed size array (array size is always 1000 no matter what)

I said the function was o(1) since we are iterating a fixed value no matter what but they insisted 0(n).

Am i wrong? Isnt it only o(n) if at worst case we fully iterate an unknown n?