We propose a method to realize orthogonal four-channel multiplexing electromagnetic orbital angular momentum (OAM) emitted from a shared aperture designed by leaky-wave holographic metasurfaces. Based on the theory of the holographic reconstruction, the hologram can be synthesized by the interference between the surface plane wave and the leaky wave carrying OAM. By combining with only two holograms in one aperture, four-channel OAM radiations can be realized based on the proposed method. Since the spatial OAM transmission is excited by transforming the surface wave, the holographic metasurfaces have the real low profile. The simulate results show good agreements with the theoretical design.