[en] The operating characteristics of a low Q (Q_cav=225), 8 GHz gyrotron oscillator operating in TE^o₀₁₁ mode and submitted to various mismatched loads are reported. Under matched conditions, output power up to 310 kW (η=35%) and maximum efficiency up to 43% have been measured. In general, power reflection from loads with different phase and amplitude leads to an output power decrease. Too high reflections cause mode switching (TE^o₁₁₁ to TE⁰₀₁₂) or even arcing inside the tube. The effect of power reflection is seen to increase rapidly with current and output power. Nevertheless, we have observed that, as predicted by the calculations, the maximum output power is not reached under matched conditions but with a specific value of the complex reflection coefficient. (author) 6 figs., 2 tabs., 13 refs