We report high level ab initio calculations (treating correlation by second order Moller-Plesset perturbation theory, MP2) of a five-dimensional normal coordinate subspace of the potential and electric dipole hypersurfaces of the C-s conformer of dideuteromethanol, CD2HOH. Accurate vibrational variational calculations are carried out using a discrete variable representation (DVR) for the five anharmonically coupled modes (three coupled CH stretching and bending modes and the OH stretching and high frequency OH bending mode). The overtone spectra of the OH chromophore are calculated and analyzed in detail with respect to their anharmonic resonance dynamics leading to short time intramolecular vibrational redistribution (IVR) via the close resonance coupling of 5 nu(OH) (5 nu(1)) with 4 nu(OH)+nu(CH)(4 nu(1)+nu(2)), as previously observed and assigned experimentally. While the assignment of the resonance is confirmed by the ab initio calculation, a sequence of calculations including various subspaces (two-dimensional to five-dimensional) lead to the conclusion that the resonance contains important contributions from coupling to the various bending modes, not just involving the CH- and OH stretching modes. Furthermore, even in the two-dimensional subspace the effective coupling constants k(1112) and k(1222) characterizing the resonance are not identical with the anharmonic potential constants C-1112 and C-1222 in the Taylor expansion of the potential, but rather an expansion to sixth order is needed to describe the resonance quantitatively. A similar conclusion holds true with other low order perturbation expansions of the resonance coupling, involving sequences of cubic couplings to other modes. We furthermore predict important resonances between OH stretching and OH bending also involving CH bending modes, which contribute to IVR at higher levels of excitation.