Does a domestic microwave work by emitting an electromagnetic wave at the same frequency as a OH bond in water? I was told once that microwaves work by exciting water molecules in food. Also that this worked because the frequency in the microwave was the same as that in the bond between Oxygen and Hydrogen in the water molecule. This similar frequency match lead to the molecule being energised, releasing heat into the food. 
My question is - how do microwaves work? Does any of this 'hearsay' have any basis in scientific fact?
 A: Your question comes down to whether the EM absorption is a resonant process or not, where resonant means it corresponds to the energy of some excitation of the water molecule. The answer is that it is not a resonant process. Microwave ovens operate at 2.45GHz but the lowest energy transitions of water molecules are rotational transitions, which have energies in the 100GHz to 1THz range. The energy of the photons in a microwave oven are too low for any resonant absorption.
Google for details of the rotational spectrum of water. I found examples here and here.
The EM radiation from the oven makes dipolar molecules within it line up with the electric field. As the field oscillates the water molecules change direction (at 2.45GHz). In liquid water the molecules interact strongly and exchange energy with each other, so the energy of the flipping motion gets transferred to translational energy of the water molecules i.e. heat. Because this is not a resonant process changing the microwave energy by small amounts (up to an order of magnitude) won't make a lot of difference to the heating. As some of the comments have mentioned, this process is called dielectric heating.
