State Rep. John Sullivan surprised many political observers by getting more votes Tuesday than Keating, who was favored in early polls and built a large campaign war chest. Sullivan received 46 percent of the vote and Keating 31 percent.
"We're overwhelmed. We were just hoping to make the runoff," said Sullivan.
"When this race started, we weren't considered much of a threat, but we knocked on doors for seven months.
Keating was pleased to make the runoff.
"I don't think you ever know anything until the last vote is counted," she said. "I intend to work hard and I intend to win."
Sullivan needed 50 percent of the vote to win the nominatin outright and avoid the Jan. 8 runoff.
The winner of the Republican showdown will face Democrat Doug Todd, a former Tulsa school board member, in the general election Feb. 12. He won his nomination with 89 percent of the vote.
The special election was called after four-term Rep. Steve Largent, a Republican, announced earlier this year he would resign Feb. 15 to run for Oklahoma governor next year.
When Keating entered the race, she was called a front-runner, holding a 2-1 advantage over Sullivan in polls. She raised $820,000 in campaign contributions, twice that of her opponents.
Although Keating campaigned alongside her husband in two successful gubernatorial campaigns, the governor did not take an active role in her race until the day of the primary, when he called her supporters to urge them to vote.