Existing techniques for measuring oxygen concentration in blood heavily relies on non-invasive transmission-mode pulse oximetry - a ratiometric optical sensing method, where light absorption in oxygenated and deoxygenated blood is interpreted to a person’s oxygen saturation (SpO2). Since transmitted light through tissues is used to generate the signal, transmission-mode pulse oximetry is restricted to only tissues that can be transilluminated, such as the ear and the fingers. Here, we present a reflection oximeter, which uses printed organic light-emitting diodes (OLEDs) and organic photodiodes (OPDs) to sense reflected light from tissues to determine the oxygen concentration. Using the reflection-mode, the sensor can be used beyond the conventional sensing locations. We used the reflection-mode sensor to measure SpO2 on the forehead with 1.1% mean error. We also demonstrate a method to determine oxygen saturation in the absence of pulsatile blood. Additionally, printing techniques are utilized to fabricate the sensor on flexible plastic substrates, making the sensor both comfortable to wear and efficient at extracting high-quality biosignal.