Resonant interactions of a coherent field with an anharmonic oscillator are classified based on the established analogy with multiphoton interactions of light with atoms. It is shown that interference interactions in which the absorption of a single coherent field quantum induces a multiphoton transition in the anharmonic oscillator are superpositions of resonances of the “cascade” and “lambda-V” types, and a general description of these resonances is given. For the third- and fourth-order nonlinearities, the general expressions are applicable to a resonant process with the absorption of a single photon and the double excitation of the oscillator. Resonant processes of the second harmonic generation and “rectification” of radiation are predicted and described; experimental investigation of these processes will provide information about anharmonicity parameters in the proposed model.