Wow, I have never interviewed at a company that asked for formal rules of big-O. And of course most professionals at those same companies (you've heard of them)

Wow, I have never interviewed at a company that asked for formal rules of big-O. And of course most professionals at those same companies (you've heard of them) tend to use "O()" to mean "approximately" or "on the order of", as in "O(1000)"