Mutual orthogonal user channels in multiuser (MU)-multiple-input multiple-output (MIMO) systems are desirable and can be approximately obtained under independent and identically distributed (i.i.d.) Rayleigh fading assumption with a very large number of base station antennas. However, it has been shown that at millimeter-wave (mmW) frequencies, this assumption is not valid due to the limited number of multipath components and spatial channel correlation. In this paper, we examine the mutual orthogonality of a realistic 60-GHz outdoor propagation channel with practical large antenna arrays, and determine the factors deciding it based on the channel data generated by means of deterministic field prediction. The results obtained reveal relationships between mutual orthogonality, inter-user distance, number of active users, transmit array dimensions, and downlink system capacity at 60-GHz band, which are useful for designing future mmW massive MU-MIMO systems.