Ontology is the beeing of things. It means the beeing itself for example of matter and not how matter appears to human. So if you are talking about an ontological theory of xy you try to explain how xy is brought to be in the world independently from how it is perceived by us. Epistemology instead is something different. It means attempts made by human to gain knowledge about the world. As humans we have several senses to realise the world we live in and we have brain that is used to interprate stimuli in a specific way (the way is set by experiences we make as individuals during our lifetime). So the way we see the world as humans is not the state of beeing of this world. For example as humans we cannot see UV-waveleghts but they are in the world. That is why scientists - who are humans - try to find a way of gaining objective knowledge about the world that is independent of the human perception. Epistemology therefore is the theory of gaining objective knowledge. But the question is more if we - as humans - will be able at all to produce that kind of objectivity or if - as I assume - will be bound for ever on the evolutionary strings of human thinking about the world.