ontology

n [U] (philosophy 哲) branch of metaphysics that deals with the nature ofexistence 本体论; 实体论.

onerous, oneself, ongoing, onion, online, onlooker, only, only that, only too, ono, onomatopoeia, onomatopoeic, onrush, onset, onshore, onside, onslaught, onto, onto (also on to), ontological, onus, onward, onward(s), onyx, oodles, oomph, ooze, op, op (also Op), op art, op-ed, opacity, opague, opal, opalescence, opalescent, opaque, opaquely, opaqueness, OPEC,