A computer system and method providing on-demand extension of ontology in an ontology use process. A support module responds to a creation event in the ontology use process. The support module defines ontology specification for a newly created item, in the creation event, in a manner substantially free of disruption or interruption to the ontology use process. An ontology manager incorporates the defined ontology specification into the ontology during the ontology use process such that an on-demand extension of the ontology results. One example ontology use process is semantic modeling of situations, organizations, or objects of interest. Modeling and ontology specification take place hand in hand, and the ontology is extended on an as-needed and on-demand fashion during the course of modeling.