The THz spectrum is rapidly growing in importance for a variety of applications including molecular spectroscopy and high-data-rate communications that require efficient, tunable high-power sources with high spectral purity. Conventional THz backward wave oscillators (BWO), such as those available from ISTOK, offer both frequency stability and tunability. Herein, we report on fundamental simulation studies to understand the RF interaction between the ISTOK THz-class BWO slow wave structure and the electron beam that powers the device. The simulations were performed with ICEPIC, the Air Force Research Laboratory’s three-dimensional, electromagnetic particle-in-cell code. Interaction efficiency, electromagnetic field geometry supported by the slow wave structure, and mode purity are discussed.