Hypoxia refers to insufficient oxygen amounts at the tissue level unable to maintain adequate homeostasis. Severe hypoxia may occur in the absence of subjective breathlessness due to respiratory failure. Precise monitoring of low blood oxygen saturation is crucially desired, catering to the clinical requirements. However, current pulse oximeters cannot function well in monitoring peripheral oxygen saturation limited by the weak peripheral blood circulation at a low oxygen level. In this work, we propose a flexible hybrid electronic (FHE) with a compact structure and high sensitivity for conveniently monitoring hypoxia. This FHE is composed of 10-µm thickness semiconductors with different materials, functionalities, and sizes. Its performance is demonstrated by monitoring arterial blood oxygen saturation (SaO2) at the body's different arteries. The absolute error is less than 2% within a SaO2 ranging from 99% to 63%. The efficient techniques presented in this work may bring light to the next-generation flexible hybrid electronics and provide potential widespread use in research and clinical applications, especially for emergency treatment.