The most common metric for evaluating the performance of an optical communications system is the symbol error probability (SEP) versus the received optical power. In circuit-like optical systems employing binary transmission such as WDM, this SEP takes the form of the Bit Error Rate (BER) which is, in general, relatively constant. However, when considering an asynchronous multiple access Optical CDMA (OCDMA) system, the BER is not constant. Since users may enter/exit the network without coordination, the instantaneous SEP of every user can be affected by the transmissions of other users because of the non-zero cross-correlation of optical spreading codes. Therefore, the SEP of an asynchronous OCDMA system becomes time-varying and statistical in nature. In this paper, we determine how to assess the performance of such a system in terms of Average SEP (ASEP) and outage probability. When the analytical description of a problem is not available and the performance can be assessed only via simulations, we also give guidelines on how to calculate the required number of trials necessary to estimate the ASEP with a certain confidence interval.