The effects of aging on viscous fingering resulting from the displacement of Oil-in-Water (O/W) emulsions with water are analyzed. The flow is examined in a rectilinear Hele-Shaw cell which is an analogue of homogeneous porous media. The experimental measurements were conducted for two concentrations of the emulsions, 40% and 60% (v/v %) that were displaced at a constant injection rate of 0.21 ml/min. The flow instabilities were found to vary strongly with the elapsed time between the initial emulsion preparation and the start of the displacement. In particular, fresh emulsions resulted in smooth finger structures that are less complex than the aged ones which tend to exhibit strong ramified branching and needle-like fingers. Furthermore, it was found that the aged emulsions show a shorter breakthrough time and a smaller sweep efficiency than their fresh counterparts. Characterization of the emulsions revealed substantial changes in the rheological behaviour and microstructure of the emulsions with age, but no significant changes in the drop size distribution. It is speculated that as a result of aging, there is an increase in flocculation or adsorption of surfactant on the dispersed droplets, which affects the emulsions' rheological behaviour and in turn the finger structures.