Recently the development of optical wireless communication has received growing attention because of its unlimited bandwidth and simple implementation as compared to radio frequency communication application for indoor wireless data transmission. The objective of this thesis is to improve the performance of intensity-modulated direct- detection (IM/DD) optical wireless communications with single carrier frequency domain equalization (SCFDE) by using Orthogonal Space Time Code (OSTC). More attention has been paid to three SCFDE schemes due to their outstanding performances in practical cases. It is demonstrated that OSTC can be modified to be suitable to optical systems and provide an excellent enhancement to system performances. By applying 4x4 OSTC that was formally used extensively in radio frequency communications, the performance of optical wireless systems with SCFDE is to be improved remarkably. It is pointed out that there exist some differences of the application of OSTC in between radio frequency communications and optical wireless communications. The performances, such as Peakto-average Power ratio, complexity and Bit-error rate, are compared and discussed. Finally the optical system, which has the best performance, is proposed and extended with multiple LEDs and Photodiodes.