Study Comparing Different Neck Treatment Approaches in Patients With Oral Cancer Without Visible Neck Spread

The goal of this clinical trial is to learn whether selective neck dissection involving lymph node levels I-III is as effective as selective neck dissection involving levels I-IV in treating adults with clinically node-negative (cN0) oral squamous cell carcinoma undergoing curative surgery. It will also evaluate whether extending the neck dissection to level IV affects cancer control, surgical complications, shoulder function, and quality of life. The main questions it aims to answer are: * Does selective neck dissection of levels I-III provide regional recurrence-free survival comparable to selective neck dissection of levels I-IV? * Does extending the neck dissection to level IV improve overall survival, disease-free survival, or reduce regional recurrence? * Does selective neck dissection of levels I-III result in less postoperative morbidity, better shoulder function, and improved quality of life compared with selective neck dissection of levels I-IV? Researchers will compare patients undergoing selective neck dissection of levels I-III with those undergoing selective neck dissection of levels I-IV to determine whether routine removal of level IV lymph nodes provides additional oncological benefit while maintaining acceptable functional outcomes. Participants will: * Undergo surgery for oral squamous cell carcinoma with random assignment to selective neck dissection of either levels I-III or levels I-IV. * Receive standard postoperative treatment, including radiotherapy or chemoradiotherapy when indicated according to the final pathology report. * Attend scheduled follow-up visits for clinical examinations and surveillance for cancer recurrence. * Undergo assessments of shoulder function, sensory function, postoperative complications, and quality of life using validated questionnaires at predefined follow-up intervals.