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Purpose: Hypoxia mediates treatment resistance in solid tumors. We evaluated if oxygen-enhanced (OE)-MRI-derived hypoxic volume (HVMRI) is repeatable and can detect radiotherapy-induced hypoxia modification in HPV-associated oropharyngeal head and neck squamous cell cancer (HNSCC)., Experimental Design: 27 patients were recruited prospectively between March 2021 and January 2024. HVMRI was measured in primary and nodal tumors prior to standard-of-care (chemo)radiotherapy then at weeks 2 and 4 (W2, W4) into therapy. Two pre-treatment scans assessed biomarker within-subject coefficient of variation (wCV) and repeatability coefficient (RC). Cohort treatment response was measured using mixed-effects modelling. Responding lesions were identified by comparing HVMRI change to RC limits of agreement (LOA)., Results: OE-MRI identified hypoxia in all lesions. HVMRI wCV was 24.6% and RC LOA were -45.7% to 84.1%. Cohort median pre-treatment HVMRI of 11.3 cm3 reduced to 6.9 cm3 at W2 and 5.9 cm3 at W4 (both p < 0.001). HVMRI was reduced in 54.5% of individual lesions by W2 and in 88.2% by W4. All lesions with W2 hypoxia reduction showed persistent modification at W4. HVMRI reduced in some lesions that showed no overall volume change. Hypoxia modification was discordant between primary and nodal tumors in 50.0% of patients., Conclusions: Radiation-induced hypoxia modification can occur as early as W2, but onset varies between patients and was not necessarily associated with overall size change. Half of all patients had discordant changes in primary and nodal tumors. These findings have implications for patient selection and timing of dose de-escalation strategies in HPV-associated oropharyngeal carcinoma.