A theoretical study is made on electron beam-excited microwave oscillations in a coaxial transmission line, with combs placed on the inner and outer conductors. The transit channel of the structure, where an annular e-beam propagates, is filled up with plasma. Eigen waves in the coaxial plasma-filled line are studied as well as their relationship vs. plasma density. Plasma is shown to cause a considerable increase in the gain, broadening concurrently the excited oscillation spectrum. The non-linear analysis is made of oscillation amplification. Determined are the maximum longitudinal electric field amplitude and coupling efficiency for different plasma densities. Comparison of the above characteristics with similar ones for the vacuum structure points out superiority of the plasma-filled hybrid structure over the vacuum structure.