Ontology - epistemology is a discourse whose separation post-structuralism managed to problematize.
Ontology - epistemology is a methodological distinction that is maintained in order to be able to distinguish a cognitive process directed to the world from a cognitive process directed to another cognitive process.
Ontology - epistemology is a distinction that implodes in post-structuralist thought because the idea of identifying some brute ontological reality is by definition epistemology (a way of 'knowing' this thing) while the idea of knowing something is ontology in the sense that to know is to be.
Ontology - epistemology is a distinction between an investigation concerned with what are the kinds and structures of the objects, properties and relations on the one hand, and on the other, an investigation concerned with how a particular subject can posit the existence of a particular object with a particular structure, or more precisely, when is a particular subject justified in holding that a particular object with a particular structure exist.