On July 11, 2008, PSO filed a base rate case with the Oklahoma Corporation Commission (OCC) for authority to increase its rates and charges for electric utility service. PSO utilized a historic test year ended February 29, 2008, adjusted for changes that are fixed, known and measurable for ratemaking purposes through August 31, 2008.

Existing rates were established by OCC order in Case No. PUD 200600285 issued in October 2007 based upon a test year ending June 30, 2006, with updates for known and measurable changes through June 30, 2007, with an ROE of 10%.