Energy transfer from P680 to oxygen-evolving complex
You have to think the other way around: don't pay attention to where the electron from P680 is going. 

Instead of that, pay attention to this fact: after PSII absorbs a photon and triggers the loss of an electron from a P680 chlorophyll a molecule, this P680 has now an electron deficit. That is, this P680 has lost one electron.

This P680 without one electron is now called P680+. And here comes the important information, which answers your question: According to Lodish (2002),


  P680+, the photochemically oxidized reaction-center chlorophyll of PSII, is the strongest biological oxidant known. (emphasis mine) 


And that explains what (thermodynamically) drives O2-evolving complex (still according to Lodish, 2002):


  The reduction potential of P680+ is more positive than that of water, and thus it can oxidize water to give O2 and H+ ions. (emphasis mine)


I hope those terms are simple enough for you. Finally, for better understanding all this, have a look at the concept of reduction potential (which, as a college student, I believe you have to study).



Source: 


Lodish, H. (2002). Molecular cell biology. 4th ed. New York, N.Y.: Freeman.