CNN declared Santorum the victor with 34 percent. Former Massachusetts Gov. Mitt Romney and former U.S. House Speaker Newt Gingrich battled it out for second place, with more than 27 percent apiece and 1,652 of 1,961 precincts counted.
U.S. Rep. Ron Paul of Texas trailed with 10 percent of the vote.
Romney was considered the winner in the Oklahoma City metro area by CNN, but Santorum claimed the vast majority of counties in the state.
Forty of Oklahoma's 43 GOP delegates were at stake Tuesday. Three delegates are considered at-large representatives.