Analytical calculations of the spectra obtained from first-order Fermi acceleration mechanism with oblique shocks are presented. They are a generalization of the quasiparallel configuration, where the streaming instability modifies the acceleration process, and of the quasiperpendicular configuration, where the magnetic pressure is significant. We show that influence of the streaming instability on the compression ratio of the fluid is always negligible, it changes the spectal index only through the modification of the effective compression ratio of the scattering centers. The observational constraints on the cut-off frequency imply that the magnetic turbulence in extragalactic jets must be weak and most probably of Kraichnan type. The application of our calculations to the jet of 3C 273 shows that the theory of diffusive acceleration in oblique shocks works quite well with a non-relativistic flow velocity, and yields narrow ranges for the different parameters.