OsCAS (Oryza sativa Chip Annotation System) is a comprehensive web-based tool that analyzes the results of rice (Oryza sativa) microarray experiments and the potential relationships among genes. This platform is designed to facilitate the indepth exploration of microarray data with relatively high analysis efficiency. With a user-friendly web interface, OsCAS allows chip probe IDs as inputs to retrieve relevant information according to user’s designation. Public databases, such as GenBank, UniGene, Swiss-Prot, TIGR, KOME, KEGG, Gene Ontology, and miRBase, were integrated into our platform to cover gene information, protein features, metabolic pathways and regulatory factors in rice. Besides, OsCAS includes the reprocessed information from several analytical tools such as CSRDB and miRU to obtain deep insights into the primary annotations. OsCAS is time-cost-effective in annotating large sets of chip probe IDs, and can provide some precious hints for the biological experiment design. OsCAS runs on a Linux server and is accessible at http://bis.zju.edu.cn/oscas .