The aim of the paper is to establish an ontology-based case encoding tool with sufficient formalization and expansibility to assist users for organizing the case information and increasing the feasibility of the design knowledge in a case library. The tool is named Open Case Study (OCS). OCS is a formalized and expandable tool for authoring metadata of a case library and organizing them by their semantic ontology. By using the templates constructed by design experts, such as design teachers or experienced architects, OCS provides the user with explicit but adaptable guidelines for case analysis and encoding. OCS then performs the searching and mapping function provided by Open Ontology. Thus, when the user is encoding the information segments of cases, relevant knowledge chunks in the case library can then be immediately provided, such as relevant senses in similar cases, all atoms of a relevant sense, and known value ranges of a relevant property. This assists users to avoid data mistake and duplication in encoding design cases.