Why benzene takes in more delocalized electrons from its substituents even though it has six of them
Using atomic orbitals to construct molecular orbitals involves some shifting and redrawing because of overlaps. Benzene is symmetrical, so whether you draw six carbon atomic p-orbitals or squash and spread them out, you get the idea. Now, when you look at phenol, the C-O bond is unsymmetrical, so if you want to be more precise, you say that oxygen draws some electron density from the carbon thru the sigma system (because oxygen is more electronegative).

The oxygen becomes richly negative at carbon's expense, but this isn't fair(!), so it gives back a little electron density thru the pi system of molecular orbitals that it doesn't care about so much (because the pi lobes are farther from the oxygen nucleus). If you draw molecular orbitals, draw the sigma bond with more electron density toward oxygen, and draw the pi system with electron density a wee bit toward carbon.

The energy diagram of the molecular orbitals of benzene will be shifted by putting a hydroxyl on the ring. One one hand you can consider the six orbitals of the aromatic system merely being shifted in the space above the carbon ring, or, on the other hand, you could consider the three filled pi orbitals to remain essentially the same while oxygen donates some electron density to the unfilled (anti-bonding) orbitals (ignoring the sigma system, which just gets shoved around a little). 

So the bottom line is that you are not putting whole electrons into someplace new, but swelling up some electron balloons while squashing some others.