Drought has been of ongoing interest to researchers due to its significant consequences. The study of drought trends and their relationship with other atmospheric phenomena, as well as their prediction using indices, plays a key role in the planning and management of water resources. The relationship between ocean oscillation indices such as North Atlantic Oscillation and the Southern Oscillation Index and drought indices such as the Reconnaissance Drought Index and the Standard Precipitation Index is investigated in this chapter. In addition, the correlation between these indices is examined in Iran as a case study.