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This paper presents reliability comparison of opportunistic scheduling and block diagonalization (BD)-precoding in downlink MIMO systems, employing orthogonal space-time block coding (OSTBC) with multiple users. The opportunistic scheduling scheme transmits data of a user to provide the maximum effective signal-to-noise ratio (SNR), while the BDprecoding scheme transmits data for all users in a precoded form to remove co-channel interference. At first, we derive the exact analytical expressions of the average effective SNR for the opportunistic scheduling and BD-precoding schemes, respectively. Next, we derive the exact analytical expressions of the outage probability for both schemes. The diversity gain of outage probability is also quantified through asymptotic analysis. From the analytical results, the opportunistic scheduling scheme is shown to provide the effective SNR gain as well as the diversity gain, compared with the BD-precoding scheme, in proportion to the number of users. Finally, we illustrate the analytical results with corresponding simulation results.