[sqeaky]

So providing a faster sort would be a violation?

The standard doesn't seem to say "or better" here, but I know that in other places it does (or least used to say something similar).

[wnesensohn]

No, f = O(n log n) means that f doesn't grow significantly faster than n log n. That is true for f = n log n, but it's also true for f = n. The "or better" is implicit by using big-O.

Note that this wouldn't be true if the standard said that it has to be Θ(n log n).

[dietbacon]

Big O complexity is an upper bound.

If something is O(1) it's also O(n) and O(n!), since it's an upper bound.