No, it's best-case O(1), worst-case O(n) (technically O(1) since there is a constant upper-bound, but let's pretend you said it's O(n) when length is divisible

No, it's best-case O(1), worst-case O(n) (technically O(1) since there is a constant upper-bound, but let's pretend you said it's O(n) when length is divisible by 50 instead of equal to 50) and average-case O(1). Each of those functions has its own big O, without specifying which function you are talking about big O isn't meaningful.