A theoretical investigation of the interaction between a charged-particle beam and ordinarily polarized surface waves at the first two harmonics of ion and electron cyclotron frequencies propagating across an external steady magnetic field is carried out. The case of weak plasma spatial dispersion when the Larmor radii of plasma particles are much less than the penetration depth of these waves into the plasma is considered. The indicated waves are eigenmodes of the planar plasma–vacuum–metal waveguide structure, which is exposed to a steady magnetic field oriented parallel to the plasma surface. It is supposed that a cold charged- particle beam propagates over the plasma surface in the vacuum region. The beam density is less than the plasma density. Excitation of these O modes at the first and second harmonics of electron and ion cyclotron frequencies caused by the both resonant beam and dissipative instabilities is studied analytically. The dependence of their growth rates on parameters of the considered waveguide structures is also examined.