Early voting began Monday at 8 a.m. and ends at 6 p.m. at county election boards. On Tuesday, polls open for the primary election at 7 a.m. in the state's 77 counties and close at 7 p.m.
Michael Clingman, state Election Board chairman, predicted a light-to-moderate turnout of between 600,000 and 700,000 of Oklahoma's roughly 2 million voters.
Clingman said voting should be heaviest in the 5th Congressional District, which Republican Ernest Istook is giving up to run for governor.
Istook said last week he would win the GOP primary for governor without a runoff against three other candidates.
Democratic incumbent Brad Henry was in a no-contest primary with Andrew Marr Jr., who got less than 5 percent of the vote last year as a Republican.
Six Republicans vied for their party's nomination in the 5th District, advertising heavily on television to stress their conservative stands on illegal immigration and other issues.
The candidates are Lt. Gov. Mary Fallin, Oklahoma City Mayor Mick Cornett, Corporation Commissioner Denise Bode, state Reps. Kevin Calvey and Fred Morgan and Johnny B. Roy, an Edmond doctor.
Istook, 56, began running a television ad last week that targeted Henry but did not mention Istook's three GOP foes — Tulsa businessman Bob Sullivan, state Sen. James Williamson and retired engineer Jim Evanoff.
Sullivan, 60, said he could land a runoff spot because polls show a large number of undecided voters.
Istook made a loan to his campaign on Tuesday for the first time after spending more than $900,000 on advertising and other expenses. He had about $68,000 left in his campaign fund on July 10.
Sullivan loaned his campaign another $100,000 on July 14 after spending more than $1 million in his bid.
A runoff was considered a virtual certainty in the 5th District Republican race.