This article presents automatic generation control (AGC) of an interconnected two area thermal system. A maiden attempt is made to apply a Proportional Integral (PI) controller in AGC using two objective functions. Controller gains are optimized using Whale optimization algorithm (WOA) and Opposition theory enabled whale optimization algorithm (OIWOA) techniques. The results of the proposed model is compared with GSA, PSO, and GA techniques. Results revealed that the OIWOA optimized PI controller shows effective and improved results over WOA, GSA, PSO, and GA techniques.