Ontology is a branch of metaphysics that focuses on existence, being, or the philosophy of reality, and includes the ontological argument for the existence of God, and such questions as the source of the Universe.
A synonym of ontology is 'state of affairs'. Though the latter term often is used in a more flexible sense of both real and absurd imagination, to ask of the ontology of, say, roses is to ask 'what is the state of affairs which is rose-ness?'
An ontological change means a fundamental change in being, such as that effected by God according to the sacramental doctrine of Christianity.
The Meaning of an Ontological Change - Frequently Asked Questions (catholicdoors.com)