Hypoxia, an inevitable feature of locally advanced solid tumors, has been known as an adverse prognostic factor, a driver of an aggressive phenotype, and an unfavorable factor in therapies. Myo-inositol trispyrophosphate (ITPP) is a hemoglobin modifier known to both increase O2release and normalize microvasculature. Our goal was to measure the tumor oxygen partial pressure dynamic changes and timing of the therapeutic window after ITPP systemic administration. Two syngeneic tumor models in mice, B16 melanoma and 4T1 breast carcinoma, were used, with varying ITPP dose schedules. Tissue oxygenation level was measured over several days in situ in live animals by Electron Paramagnetic Resonance oximetry with implanted OxyChip used as a constant sensor of the local pO2value. Both B16 and 4T1 tumors became more normoxic after ITPP treatment, with pO2levels elevated by 10–20 mm Hg compared to the control. The increase in pO2was either transient or sustained, and the underlying mechanism relied on shifting hypoxic tumor areas to normoxia. The effect depended on ITPP delivery intervals regarding the tumor type and growth rate. Moreover, hypoxic tumors before treatment responded better than normoxic ones. In conclusion, the ITPP-generated oxygen therapeutic window may be valuable for anti-tumor therapies requiring oxygen, such as radio-, photo- or immunotherapy. Furthermore, such a combinatory treatment can be especially beneficial for hypoxic tumors.