Radio waves carrying orbital angular momentum (OAM) have been used in the wireless communication for the multiple-input multiple-output (MIMO) application due to the OAM mode orthogonality. However, many fundamental properties like the channel performance of OAM waves penetrating through specific apertures are still missing in press. Hence, this paper investigates the OAM wave based channel through an annular slot aperture in an infinite perfect conductive wall. The far field radiation patterns of OAM beams are observed before and after the annular slot aperture, which have significant changes. The electrical fields around the aperture are also visualized to understand the OAM wave penetration, indicating that the OAM phase distribution is still maintained after the annular aperture, so as to the OAM mode orthogonality. A full wave analysis of the OAM wave channel is configured using uniform circular arrays of eight dipoles for each transmitting and receiving arrays. The investigation operating at 3.5 GHz reveals that the OAM receiver can still obtain very good mode isolation under the variation of the annular aperture sizes. Furthermore, multiple receiving arrays allocated simultaneously after the aperture both able to receive corresponding OAM modes properly, paving a way for multiple-input OAM array to multiple-output OAM arrays, offerina an additional degree of freedom for MIMO systems.