Because you are changing concentrations, we can assume the cell is not under standard conditions (everything at 1M concentrations). Therefore, look to the nernst equation for the new EMF.
Ecell = E0cell - (RT/nF)lnQ
In this case, increasing the concentration of oxidant at the cathode changes the Q value. If you write out the full reaction for the cell, you can determine products and reactants and see how varying concentrations increase or decrease E from its standard value.