  The successful operation of the hard x-ray self-seeding experiment at the
LCLS opens the era of fully coherent hard x-ray free electron lasers (FELs).
However, the shot-to-shot radiation fluctuation is still a serious issue. In
this paper, high-gain, single-pass x-ray FEL schemes seeded by the narrow
bandwidth radiation signal from an x-ray FEL oscillator were proposed and
investigated, which are expected to generate high brightness, fully coherent
and stable x-ray pulse. A simple model has been developed to figure out the
temporal and the spectral structures of the output pulses in x-ray FEL
oscillator. And options using two synchronized accelerators and using one
accelerator were considered, respectively.
