The relationship between these two variables is an inverse relationship. That is, as one goes up, the other goes down. As the opportunity cost of a given behavior changes, the frequency of that behavior changes in the opposite direction.
Opportunity cost is what I give up in order to do a certain action. If the opportunity cost goes up, I have to give up more to do that action and I will become less likely to do it. If the opportunity cost goes down, I will be more likely to do that action. Thus, the relationship is inverse.