The aim of this study was to investigate the predictability of occult lymph node metastasis using maximum standardized uptake value (SUVmax) in the primary tumor on pre-treatment 18-fluorodeoxyglucose positron emission tomography FDG-PET in oral squamous cell carcinoma (OSCC) patients who were clinically node negative (cN0) before surgery. A retrospective analysis of all patients treated at the University Hospital Zurich from 2007 to 2016 for OSCC with available pre-therapeutic FDG-PET was performed. We assessed the correlation of SUVmax of the primary tumors with the presence of occult nodal disease in the neck dissection specimen (pN+). The study included a total of 71 patients. In the nodal negative group (cN0/pN0), the median SUVmax of primary tumors was 9.0 (interquartile range (IQR) 7.4–13.9), while it was 11.4 (IQR 9.9–15.7) in the occult metastatic group (cN0/pN+). The difference was statistically significant (independent samples median test, P = 0.037). In a multivariable model, the only independent predictor of occult metastatic disease for cN0 patients was a SUVmax ≥ 9.5 (P = 0.028). Further, primary tumors with SUVmax ≥ 9.5 had a significantly higher risk of local recurrence (Log rank test, P = 0.020). In conclusion, we showed that higher SUVmax (≥9.5) of the primary tumor is associated with higher occurrence of occult metastatic nodal disease and worse local survival. High SUVmax of the primary tumor may encourage clinicians towards more aggressive treatment.