One of the most important arguments against scientific realism is underdetermination, so realists should address the argument satisfactorily and give an appropriate response to it. Underdetermination divides into holist and contrastive forms; in this article, we will only deal with the contrastive form. By examining different responses, it will turn out that while they are successful in some cases, there are instances where responses are not satisfactory. Specifically, two formulations of Newtonian mechanics are examined and the examination reveals an important (...) instance of underdetermination that cannot be accounted for by most responses. We argue that the appropriate option is ontic structural realism that can resolve the underdetermination. We also should emphasize that ontic structural realism is empirical and a posteriori, meaning that the truth of its claim must be examined on a case-by-case basis. (shrink)