Background According to the AJCC third to seventh edition staging manuals (1988–2010), the presence of through cortex and/or skin invasion in oral cavity squamous cell carcinoma (OCSCC) identifies T4a tumors. The AJCC eighth edition (2018) introduced a depth of invasion (DOI) > 20 mm as a criterion for pT4a. Subsequently, a revision maintained that tumors > 4 cm with a DOI > 10 mm should be classified as pT4a. We sought to analyze the prognostic impact of the three distinct criteria identifying pT4a disease. Methods We examined 667 consecutive patients with pT3-4 buccal/gum/hard palate/retromolar SCC who underwent surgery between 1996 and 2016. pT1/pT2 ( n  = 108/359) disease were included for comparison purposes. Results The 5-year outcomes of patients with pT1/pT2/without ( n  = 406)/with tumor > 4 cm/DOI > 10 mm ( n  = 261), pT1/pT2/DOI ≤ 20 mm ( n  = 510) / > 20 mm ( n  = 157), and pT1/pT2/without ( n  = 305) / with through cortex/skin invasion ( n  = 362) were as follows: disease-specific survival (DSS), 98%/89%/79%/65%, p  < 0.001, 98%/89%/78%/59%, p  < 0.001, and 98%/89%79%/69%, p  < 0.001; overall survival (OS), 90%/79%/63%/51%, p  < 0.001, 90%/79%/63%/42%, p  < 0.001, and 90%/79%/65%/52%, p  < 0.001. In pT3-4 disease, a tumor > 4 cm/DOI > 10 mm was an independent adverse prognosticator for 5-year DSS rate, DOI > 20 mm was an independent adverse prognosticator for 5-year DSS and OS rates, whereas through cortex/skin invasion independently predicted 5-year OS rates. Conclusions All of the three criteria (tumor > 4 cm/DOI > 10 mm, DOI > 20 mm, and through cortex/skin invasion) identify high-risk patients, which should be reflected in further revisions of pT4a classification in OCSCC.