This paper reports on experimental and theoretical studies of a fundamental mode, large‐orbit gyrotron oscillator designed to produce high‐power radiation in the rectangular TE10mode at about 650 MHz. A basic linear theory for design and operation is presented and experiments confirm its applicability. Results of initial experiments have demonstrated 40±4.2 MW of microwave radiation in pulses equivalent in duration to the electron beam, corresponding to an efficiency of about 16%. In some cases, much greater power levels of 100±20 MW were observed at the expected frequency but in pulses of very short duration. In both cases, the observed frequency and mode pattern were consistent with operation in the fundamental mode. Experiments also demonstrated that the observed radiation was linearly polarized, as expected.