We compare outcomes in two large-scale contemporaneously treated HPV-positive (HPV+) oropharynx cancer (OPC) cohorts treated with definitive radiotherapy/chemoradiotherapy (RT/CRT). p16-confirmed HPV+ OPC treated between 2007 and 2015 at PMH and DAHANCA were identified. Locoregional failure (LRF), distant metastasis (DM), and overall survival (OS) were compared. Multivariable analysis (MVA) calculated adjusted-hazard-ratio (aHR) with 95% confidence interval (95% CI), adjusting for cohort, age, gender, performance status, smoking pack-years, T-category and N-category and chemotherapy. Compared to PMH (n = 701), DAHANCA (n = 1174) contained lower TNM-8T-categories (T1-T2: 77% vs 56%), N-categories (N0-N1: 77% vs 67%) and stages (stage I: 63% vs 44% (all P < .001). PMH used standard-fractionation CRT in 69% (481) while 31% (220) received hypofractionated or moderately accelerated RT-alone. All DAHANCA patients were treated with moderately accelerated RT; 96% (1129) received nimorazole (NIM) and 73% (856) concurrent weekly cisplatin. DAHANCA had shorter overall-treatment-time (P < .001), lower gross tumor (66-68 vs 70 Gy) and elective neck (50 vs 56 Gy) doses. Median follow-up was 4.8 years. DAHANCA had higher 5-year LRF (13% vs 7%, aHR = 0.47 [0.34-0.67]), comparable DM (7% vs 12%, aHR = 1.32 [0.95-1.82]), but better OS (85% vs 80%, aHR = 1.30 [1.01-1.68]). CRT patients had a lower risk of LRF (aHR 0.56 [0.39-0.82]), DM (aHR 0.70 [0.50-1.00]) and death (aHR 0.39 [0.29-0.52]) vs RT-alone. We observed exemplary outcomes for two large-scale trans-Atlantic HPV+ OPC cohorts treated in a similar manner. Concurrent chemotherapy was a strong, independent prognostic factor for all endpoints. Our findings underscore the need for a very careful approach to de-intensification of treatment for this disease.