Big O can mean a lot of things. People often only think of BigO in terms of cpu cycles but it can be applied to memory and network just as easily.

Consider a piece of code that grabs a set of records and then loops over them to get an association for each one. I've seen this kind of code in code reviews from juniors. its a good starting point where I start showing them how to rewrite their sql call to get the associated record via join so it goes from O(n) network calls to O(1)