Thomas R. Gruber and Greg R. Olsen. (1994). An ontology for engineering mathematics. In J. Doyle, P. Torasso, and E. Sandewall (Eds.), Fourth International Conference on Principles of Knowledge Representation and Reasoning, Gustav Stresemann Institut, Bonn, Germany, Morgan Kaufmann, 1994.
Possibly the first refereed publication of an AI ontology, explicitly called out as an ontology.