We studied the operation of an oversized backward wave oscillator driven by an electron beam with energy less than 100 keV for terahertz radiation. Cylindrical slow wave structures (SWSs) with rectangular corrugations and annular electron beam are used to generate broadband terahertz radiation. The starting conditions for intense terahertz radiation are examined by varying the beam parameter and the SWS length. Operation frequencies in the range of 0.26-0.34 THz and 0.31-0.39 THz are obtained using two types of SWSs with different corrugation amplitudes. The obtained maximum output power is at the kilowatt level. Characteristics of the beam instabilities in the terahertz-SWSs are examined by comparison between the experimental and numerical results.