  We study the weakly stable hyperbolic boundary value problem with a large
zero order oscillatory coefficient. This problem is related to linearized
problems in the study of Mach stem and vortex sheets. We wish to establish a
uniform energy estimate with respect to {\epsilon}, which is needed in
mentioned applications and justification of geometric optics solutions, but the
zero order oscillatory term gives rise to great obstacles. In this paper we
obtain positive results in the small/medium frequency region by adapting the
approach of Williams to a more general situation than the one treated there. We
also show that it is possible to construct high order approximate solutions by
the method of geometric optics for those systems without any restriction on
frequencies.
