In higher education in Taiwan, mandatory general English courses like Freshman English adopt ability-grouping practices to assign students to classes of different proficiency levels. However, little research has explored the efficacy of ability-grouping criteria and standardized the language-proficiency description for general English courses of different proficiency levels. Thus, this study recruited 806 Taiwanese undergraduates from Freshman English classes of advanced, intermediate, and basic proficiency levels to take the Oxford Online Placement Test (OOPT), and compared their scores on the Joint College Entrance Examination English subject (namely the General Scholastic Ability Test, GSAT) with OOPT. The findings indicated that the ability-grouping criterion was effective to classify students into three groups of different proficiency levels. A positive relationship between the GSAT and OOPT scores was identified. A clear trend, but with some overlapping correspondence, was displayed between the class levels and Common European Framework of Reference levels derived from the OOPT scoring report. Pedagogical implications and suggestions for future studies are provided.