We propose a threshold-based multiple optical signal selection scheme (TMOS) for free-space optical wavelength division multiplexing systems. With this scheme, we can obtain higher spectral efficiency while reducing the possible complexity of implementation caused by the beam-selection scheme and without a considerable performance loss. To characterize the performance of our scheme, we statistically analyze the operation characteristics under conventional detection conditions (i.e., heterodyne detection and intensity modulation/direct detection techniques) with log-normal turbulence while taking into consideration the impact of pointing error. More specifically, we derive exact closed-form expressions for the outage probability, the average bit error rate, and the average spectral efficiency while adopting an adaptive modulation. Some selected results show that TMOS increases the average spectral efficiency while maintaining a minimum average bit error rate requirement.