Semantic interoperability is one of the key challenges in integrating disparate design software systems. Ontology provides formal and explicit specifications to describe semantics of design information for interoperability. This paper examines how semantic metadata from proprietary design information models can be captured in semantic enhancements, while data meanings formalized in ontologies to support semantic interoperability of product design applications. The paper demonstrates the proposed approach with use scenarios in integrating and sharing design semantics across mechanical, electrical, and optical CAD applications and other design information systems.