Free space optical (FSO) communication system presents a viable and innovative technology for enabling point-to-point wireless data transmission across the atmosphere. Atmospheric turbulence and pointing errors impede optical signal propagation in free space, causing the signal to fade at the receiver and degrading connection performance. In order to provide a comprehensive assessment of the partially coherent FSO links, which are used commercially to enhance and support various wireless communication needs, one of the primary performance indicators that has been identified and utilized in the proposed research is the probability of outage. In this paper, we investigate the probability of outage of the FSO system against transmit optical power under clear weather and moderate rain, which is carried out through simulation. Relevant parameters such as link distances, data rates and aperture diameters are considered to optimize the transmitted optical power for maximizing the system performance subject to an outage. It is evident that, shorter link distances and raising aperture diameter in an optimized way can reduce the consumption of transmit optical power, while maintaining desirably low probability of outage.