Lets suppose O atom has charge -a and H atom +a charge. How the charge will differ on each individual atom if the bond strength ( bond length ) decreases or increases.
Qualitative MO theory suggests that for a C2v H2O molecule, the O s and pz orbitals (a1 symmetry) interact with the in-phase combination of H s orbitals (these are formally vacant and protonic because of their higher energy). The interaction causes an inner electron shifting from the oxygen atom, which is evidently smaller at larger O…H distances, allowing the oxygen atom to remain more negative upon O-H elongations. A similar consideration applies also for the interaction of the b1 type between the filled O px orbital and the out of phase combination of H s orbitals. In this case, the effect of the O-H elongation (but also the H...H one) induces a less dramatic electron density redistribution. In fact, for a longer H...H distance, the b1 combination of the H s orbitals is less destabilized and the the reduced energy gap with O px compensates in part the reduction of the electron transfer upon the elongation.
As an indication, a simple extended Hueckel calculation shows that the O charge varies from -0.82 to -0.87 in the O-H range 0.95-1.15 Å. Also, the H-O-H angle may have an effect, although I believe that the one indicated for the distances is prevailing.
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