A multiuser multiple-input-multiple-output (MIMO) system with orthogonal space-time block coding (OSTBC) is analyzed for the uplink of a wireless communication system in a Rayleigh fading environment. In the first part of this paper, a simple and exact closed-form expression for the outage probability of the signal-to-interference-and-noise ratio (SINR) is derived at the input of the base station (BS) receiver by making the following two assumptions: 1) All the users transmit their data by using the same OSTBC; and 2) the users are power controlled by the same BS so that the interfering users are requested to transmit with the same power. In the second part of this contribution, the outage probability of the signal-to-interference ratio (SIR) is calculated at the output of the BS receiver, which, in our case, is a spatial matched filter. To be able to analytically solve the latter problem, the presented analysis is restricted to the case of a single interferer and a 2 2 MIMO system with Alamouti coding. Monte Carlo simulations are carried out to verify the proposed analytical expressions for the outage probability.