In this paper, a rigorous performance analysis of optical CDMA (OCDMA) systems is presented for three different structures. These structures are on-off orthogonal (OOO), on-off orthogonal with single optical hard limiter (OOO-SHL) and on-off orthogonal with double optical hard limiters (OOO-DHL). For each of the above schemes two types of receiver structures is evaluated. The approximation for an avalanche photodiode (APD) output where the effect of APD noise, thermal noise and interference are included. Also, the numerical results have been plotted.