We improve the performance of the K-band oversized backward wave oscillator. The beam is annular and weakly relativistic. The beam voltage is less than 100 kV and the beam current is less than 500 A. The operation frequencies are in the range of 23-27 GHz. The electromagnetic field of the K-band BWO concentrates in the vicinity of the SWS wall and the electron beam should be propagated within a few mm from the wall. The uniformity of annular beam is also very important for the efficient beam coupling. First, we modify the oversized SWS so that the beam interaction point is shifted to the point. Secondly, we improve uniformity of the annular electron beam and the output power increases up to about 500 kW level.