Pulse oximetry is a common measure of patient health due to the correlation between peripheral oxygen saturation and arterial oxygen saturation. Current clinical grade pulse oximeters operate in transmittance mode and therefore must be placed on extremities such as the fingers, restricting patient mobility. Reflectance mode pulse oximeters are widely used in consumer applications, but lack the accuracy and precision required in clinical settings. In this paper, a novel wavelength-division differential detection technique is proposed which allows for a microwave-sensing based approach to reflectance mode pulse oximetry. The theory of microwave wavelength-division differential detection is given, then evaluated using a full-wave simulation of a wearable setup. The theoretical results demonstrate that wavelength-division differential detection produces a signal proportional to changes in the blood's dielectric characteristics but is dependent on the distance from sensor to target. Full-wave results confirm that wavelength-division differential detection may provide an avenue for a more accurate reflectance mode pulse oximetry measurement using microwave near-field sensing.