Objective The purpose of our study was to investigate body mass index (BMI) as a prognostic factor and to examine the relationship between pretreatment BMI and oral squamous cell carcinoma (OSCC) outcomes in northern Chinese patients. Methods This retrospective study enrolled 633 patients with OSCC who underwent surgery between 2004 and 2014. Results Most baseline characteristics (gender, sites, smoking history, comorbidity, hypertension, T stage, clinical features, perineural invasion, flap reconstruction) were differentiated by BMI groups. Overall, the Kaplan-Meier curves indicated no significant relationship between BMI and disease-free survival (DFS) or disease-specific survival (DSS). Interestingly, obese patients exhibited higher risks of recurrence and death than normal-weight patients (DFS: HR: 1.86, 95% CI: 1.16-2.96; DSS: HR: 2.04, 95% CI: 1.08-3.86). However, postoperative complications occurred more frequently in underweight patients than in normal-weight patients. Conclusions Obesity may contribute to a higher recurrence rate and a worse prognosis in OSCC patients than in normal-weight patients in northern China. However, underweight patients have a higher risk of postoperative complications.