Based on a brief analysis of the emerging challenges and development strategies for digital libraries facing the convergence of the cyberinfrastructure, Semantic Web, and Web Services, the author advocates a new paradigm of digital libraries that encompass knowledge content, context, and communities, using as an example the three-tiered Digital Library Framework of the Chinese Science Digital Library strategic plan. Within this framework, an Ontological Service Layer (OSL) plays the central role as a semantic glue for knowledge-based integration of digital resources, services, and processes. Further analysis is given to the functional requirements of the OSL and a high level architecture of the OSL is presented. An exploration of the structure, faceted ontology components, and possible construction tools of the underline Integrate Ontology System of the OSL is then made to illustrate approaches to implement the OSL.