This paper describes the effects of outlet boundary conditions on the one-dimensional linear analysis of combustion oscillation in a DLE (Dry Low Emission) gas turbine. Ordinarily, the acoustic outlet condition of the combustor is treated as open end in conventional combustion oscillation analysis. However, in a gas turbine engine, this condition is over-simplified. In this study, an acoustic impedance of the combustor-connected plenum was introduced in the one-dimensional analysis to improve prediction accuracy. The acoustic impedance was measured by using the two-microphone method and approximated by a rational function of angular frequency. An analysis was carried out for the test gas turbine combustor with a scroll-shaped plenum. 285 Hz combustion oscillation occurred at 95 % load in the test gas turbine. The new boundary condition with acoustic impedance predicted an oscillation at 284 Hz, while the open-end condition did not indicate any oscillation occurrence. Parametric studies were then carried out to expand the margin for operation without combustion oscillation. Flame lifting and extension of LP (Lean premixed) burner were proved to be appropriate for the test gas turbine. Experiments using the new LP burners with these modifications were performed; operational margin increased more than 10 % of load. It was concluded that consideration of the acoustic impedance of combustor-connected plenum yields good results for the DLE combustor.