I think that it's a question of how difficult it is to hold the entire idea in your head. The utility of OOP is in breaking down a difficult idea into manageable parts; if you can commandingly capture the entire thing in a thought, OOP is overkill. Write your program and be done with it; tweak when needed. If you find yourself backtracking and needing to map things out on paper, though, OOP is probably a much better fit.

So my answer is basically, "Can you explain it in the elevator?"