I wonder if your uncertainty is over what ontology is. One's ontology is one's theory of what sorts of objects have reality. For instance, most everyone agrees that physical objects are real--perhaps, in some ways, Buddhist, Plato, and Cartesian skeptics might be excepted from this group. Therefore such people have an ontological commitment to the reality of physical objects. Mathematical realists have an ontological commitment to the reality of numbers, so that they exist in the way that (most people assume) physical objects do.
I doubt that one can effectively characterize the sorts of theses that would entail ontological commitments. Obviously, theses about the ways in which we may make inferences based on evidence can carry with it theological ontologies or their lack. On the other hand, it is debated whether correspondence theory of truth applied to mathematical objects implies an ontological commitment to numbers.