The rate of oxygen delivery is a very important factor in aerobic processes. In many cases, this determines the efficiency of processes such as biological wastewater treatment. The availability of oxygen is essential for sustaining the growth of microorganisms necessary for degrading the organic matter. In this work, a continuous, re-circulation, constant hold-up plunging free jet-loop system was designed to study the effects of design parameters on the oxygen mass transfer. This was investigated using three different nozzle diameters 6, 10 and 14 mm. For a fixed value of diameter, the distance between the nozzle mouth and the free liquid surface was varied to cover the range 14-26 cm. The experimental results showed significant effects of these two parameters. The oxygen mass transfer coefficient was found to increase with either increasing the nozzle position or decreasing its diameter.