The Phase III study will include 174 patients with locally advanced oropharynx carcinoma, receiving all arc-IMRT (Intensity-Modulated Radiation Therapy) (70 Gy) with concomitant systemic therapy. Two arc-IMRT treatment arms will be compared: one "standard" arm based on the use of a single pre-treatment planning and one "experimental" arm (adaptive RT) based on a weekly replanning to spare the salivary glands. The main objective is to increase by 25% the salivary flow (Parafilm) 12 months after RT thanks to adaptive RT, while not decreasing local control. The secondary objectives are to increase the salivary flow (scintigraphy), reduce xerostomia, acute and late toxicities (Eisbruch questionnaire, MDAS-HN, v.4 CTCAE), while maintaining local control (stopping rule of the trial if difference>15%). 174 patients will be included in 6 French centers for 2 years and followed for 2 years. The HPV (Human Papillomavirus) status will be identified and the tumors frozen. A central IMRT QA (Quality Assurance) will be performed.