Since Oct. 2010, statewide average precip for Oklahoma is 26 inches below normal, according to Gary McManus, Oklahoma Climatological Survey
The current drought period dates back to October 2010. If you look at the rainfall statistics going back that far, 28 months ago, you see the driest such period on record with a statewide average of 56.77 inches. That sounds deceptively impressive, since the normal for that time frame would be 83 inches. So since October 1, 2010, the statewide average deficit is 26 inches. The statistics are obviously much worse for western Oklahoma, which has seen significant drought remain in place since that time.
Source: Gary McManus, Oklahoma Climatological Survey.