We demonstrate that energy from energetic electrons upstream of interplanetary (IP) shocks are able to couple directly to the o-mode radio branch via the relativistic cyclotron harmonic resonance interaction, thereby creating the Type II radiation observed from IP shocks. The conditions required for the electron/wave resonance are that the wave frequency, f, lies near the local plasma frequency, f_p, and near (but not exactly at) the cyclotron harmonics, mf_c, where m is the cyclotron harmonic number. We compare the details of this relativistic electron cyclotron harmonic theory with observations in a Type II source region, with good agreement in many areas including the prediction of "Zeeman" splitting of the Type II emission. Based on the character of the high-resolution emission spectrum, we can narrow the Type II source location to the IP shock foot region and place some general conditions on shock topology required for Type II emission.