The paper presents voltage and reactive power control in the Main Interconnected Transmission System (MITS) in the Sultanate of Oman. The objective is to improve voltage profile at substations supplying distribution companies and directly connected bulk customer. An objective function is used to maximize reactive power margin of generating units to maintain as much as possible reserves on different generators. The control parameters include existing voltage and reactive power control means, e.g. settings of generators’ excitation/AVR, transformer tap-changing, and capacitor bank switching. The constraints include operational limits of the above control devices in addition to allowable limits of busbar voltages, loading of power equipment, and power exchange. The optimal power flow technique is used to determine the optimal settings of the control facilities. The main advantage of this technique is that its implementation does not require any new investments. A model of the MITS is developed and simulation results are presented to show the improvement in voltage profile. Practical measurements of power factor and voltage at substations are also presented to show the improvement in voltage.