In what domain do questions about the nature of knowledge fall epistemology or ontology? How do you distinguish the two fields?
I don't actually understood what you're trying ask here, but I can point out that 'questions about the nature of knowledge' ARE epistemological questions per se. 'Do you need any ontology to answer epistemological questions?' - I think that's where you are trying to get to. Well, ontology is all about what things are and it seems that in many cases epistemologists do need to do some ontology [and vice-versa]; metaphysicians do loads of epistemology as well. Just a few examples, 'Knowledge and its Limits' and 'Modal Logics as Metaphysics' from Timothy Williamson, 'Formal Epistemology' Von Hendricks, etc.
A quick google search would have done it, so next time, please don't be lazy.
Ontology is the philosophical study of the nature of being, becoming, existence, or reality, as well as the basic categories of being and their relations.
Epistemology is the branch of philosophy concerned with the nature and scope of knowledge and is also referred to as "theory of knowledge". It questions what knowledge is and how it can be acquired, and the extent to which knowledge pertinent to any given subject or entity can be acquired.