Accurate evaluation of oral tissue defects following oncological surgery is necessary for the subsequent reconstruction. However, there is currently no effective classification system for oral defects in the clinical setting. The present study therefore developed a clinical classification system for the evaluation and reconstruction of oral defects. A retrospective cohort study was performed. A two-dimensional classification system based on coronal computed tomography/magnetic resonance imaging was developed and validated by 145 cases with oral defects. Oral defects could be classified into 6 types (I-VI) horizontally and 2 classes (a and b) vertically. The proportion of the various types was as follows: Type I, 35.9%; type II, 21.4%; type III, 23.4%; type IV, 4.8%; type V, 2.1%; and type VI, 12.4%. Among them, 91 cases (62.8%) were class a and 54 cases (37.2%) were class b. Type Ia-Va represented the unilateral 1-5 subsites involving superficial oral defects without mandibular continuity destruction (88 cases, 60.7%). Type Ib-Vb (+M) represented the unilateral 1-5 subsites involving deep oral defects with segmental mandibular continuity destruction (38 cases, 26.2%). Type I-V (+S) represented the unilateral through and through oral defects with cheek skin involvement (10 cases, 6.9%). Type VI represented bilateral oral defects (18 cases, 12.4%). The present classification system for the evaluation of the oral defects was simple and practical, and could identify the common types of oral defects and guide the reconstruction.