Designs and implementation of a novel tunable 250 GHz gyrotron oscillator for Dynamic Nuclear Polarization (DNP) experiments with >;10 W output power over most of a 3 GHz band and >;35 W peak power are shown. The bandwidth and power can generate a substantial THz field over the band used for DNP experiments, without the need for sweeping the NMR magnetic field. Substantially improved tunability is achieved by implementing a long (23 mm) interaction cavity that can excite higher order axial modes by changing either the magnetic field of the gyrotron or the cathode potential. This interaction cavity excites the rotating TE _5,2,q mode, and an internal mode converter outputs a high-quality microwave beam with >;94% Gaussian content. The gyrotron was integrated into a DNP spectrometer, resulting in 25% improved DNP enhancements.