Prognostic imaging biomarkers to assess tumor hypoxia remain a Holy Grail. We present evidence that T1-weighted signal response to an oxygen breathing challenge before a single high dose of radiation is related to long term tumor control. Specifically, Dunning prostate R3327-AT1 tumors showing a small signal response showed poor control for relatively lower doses (30-50 Gy). Meanwhile, higher doses overcame the radio resistance, which we associate with hypoxia. A large BOLD response (ΔR2*) was associated with poor outcome irrespective of radiation dose. These results provide further evidence for the potential utility of oxygen sensitive MRI in guiding radiation therapy.