Dissolved oxygen (DO) is an important indicator for water quality and is also used in point of care applications.  Fluorescence signal from Ruthenium (Ru(dpp)₃²⁺ ) based fluorophores can be quenched by oxygen such that the reduction of fluorescence signal is proportionally to oxygen concentration. This mechanism allows an optical method to measure DO in liquid.  Although optical DO measurements offer high sensitivity and stability, they are also expensive, bulky, and difficult to use due to its complex design and requirement for specialized optical instruments such as light sources and spectrometers. Recently, the advances in microfabricated optical component including waveguides, lenses and mirrors, light sources, and photo detectors enable the integration of optical sensing and microfluidics sampling platform. We report the development of a total internal reflection assisted (TIRA) optical DO sensor for sensitivity enhancement.  The excitation light is conducted down the water channel direction in the microfluidic device, while being confined within glass slide by total internal reflection. In addition to steady state intensity measurements, oxygen quenching is measured in the frequency domain using modulated light sources and synchronized detection systems. Experimental results show that optical sensitivity can be increased up to an order of magnitude in TIRA sensors. These results suggest the potential of TIRA scheme as a sensing and characterizing platform for optofluidic sensors.