[eru]

Whether you call that O(1) or O(n) depends on the level of abstraction you want to talk about, and what you want to analyse.

For example, in some sense, anything you can do on a finite computer is O(1) (or alternatively, it's an infinite loop). But that is a very boring sense, and seldom useful for analysis.

There's a formal definition for big-O notation, but most of the parameters to that formal definition are only implied in casual discussions.