The traditional approach to presenting idioms relies mainly on teachers' or materials writers' judgement, one-by-one and quite incidentally; and the existing teaching materials and references for idioms are mostly intuition-based. However, a more recent approach to better teaching and learning idioms is to present them under categories of their common themes and topics. Corpus linguistics can be of much contribution through helping the design and development of more authentic and systematic materials using comprehensive corpora which are typically the best representatives of the target language. In this connection, the present study aimed at searching for the thematic index of 1506 idioms under 81 categories at the end of the Oxford Dictionary of Idioms in the largest freely available corpus, i.e. the Corpus of Contemporary American English (COCA), composed of more than 520 million words. To this end, we used a manuscript in PHP written by a professional computer programmer especially for this purpose. The findings yielded a list of idioms sorted based on their frequencies under their theme-based categories. To focus on the more frequently-used idioms of various themes in real contexts, materials designers, teachers, and learners of English can benefit from this idiom list in textbooks and classroom activities.