Consideration concerns a two-dimensional system of molecules fixed by an adsorption potential at lattice sites of a square lattice at a definite angle theta to the surface normal. Among lateral intermolecular interactions including quadrupole, dispersion, and repulsive, the former are regarded as predominating. At small theta, the Hamiltonian of the system is shown to reduce to that of the thoroughly studied two-dimensional dipole system with renormalized interaction constants. The hierarchy of ground-state orientational phases depending on the angle theta is established in two particular cases: for free azimuthal rotation and for four discrete orientations of projections of long molecular axes along square-lattice axes. The latter case is included in the diagram of temperature phase transitions. The results obtained adequately describe the properties of the 2x1 phase of CO/NaCl(100). The structure of the observed ground state and the orientational phase transition temperature are reproduced correctly. The spectrum of orientational vibrations calculated for this system is characterized by the band half-width which is consistent in the order of magnitude with those found from experimental temperature dependences of spectral line shifts and widths for local C-O vibrations dephased on orientational vibrations.