The linear theory of electromagnetic radiation from a plasma-loaded sinusoidally corrugated backward wave oscillator driven by intense relativistic electron beam has been developed and analyzed numerically as well as analytically. A beam-plasma waveguide model has been proposed and studied analytically with an aim to give a guideline to achieve an optimally designed backward wave oscillator in the X-band frequency range. First, the dispersion relation for TM01 mode in absence of electron beam has been analyzed numerically for different structure-size parameters and plasma density. Then, developing the analytical dispersion relation, the effects of variation of structure parameters and plasma density on the spatial and temporal growth rates for TM01 mode are investigated. Here, the analytical investigation, have been carried out by absolute instability analysis. It is observed that the variation of structure-size parameters and filled-plasma density modify the dispersion curves of the backward wave oscillator significantly and affect the beam-wave interaction frequency, the temporal and spatial growth rates and thus the signal strengths considerably.