Ab initio molecular orbital theory is used to determine if the molecular ion HOC+ is linear (as are the isoelectronic species HCN, HNC, HCO+, etc.), or if it is quasilinear. Near the Hartree-Fock limit the molecule is either linear, or very close to linear. Electron correlation favors the linear geometry, leading to the unequivocal prediction of a linear molecule. Detailed comparisons between HOC+ and isoelectronic HNC show an apparent lack of convergence in the bending potential for the former which is remedied by the addition of f functions to the basis set. The HOC+ potential energy surface is computed in the bending and bend-stretch coordinates and fit to an analytical function. Use of this function to compute the rotation-vibration energies results in improved agreement with experiment relative to previous potentials by nearly two orders of magnitude, as documented in the accompanying paper. © 1987.