In the reaction

Question

2H20(aq) --> O2(g) + 2H20(l)
why doesn't increasing the pressure of oxygen have any effect on the reaction rate?

Balance the equation first. What you have isn't balanced. Frankly, it doesn't look possible. You have two mols H2O on each side. Take them out and the equation reads:
??? ==> O2 which means you get oxygen from nothing. Can't do that.