Wireless communication of radio waves carrying orbital angular momentum is investigated in this paper, to explore the impact of the orientations of transmitting and receiving arrays. As an example, two circular arrays, each containing eight dipoles, are taken as a transmitter and a receiver of radio waves carrying different OAM modes. A mixed-mode matrix, taking consideration of all possible OAM modes, is used to interpret the communication performance. Thanks to the advantage of the mixed-mode matrix, the received power of an OAM mode is a combination of all generated OAM modes at the transmitter. The orientations of the transmitter and receiver are studied using the method of moment (MoM) to reveal their dependencies of OAM-based communication. Based on the results, a general guideline with regard to array orientations for the optimization of OAM-based communication is derived.