With all precincts reporting, Balderson, an OH state senator who closely linked himself to President Trump - appearing with him at a rally last weekend - held a 0.9 percent lead over O'Connor, a county recorder and moderate Democrat.
Republicans' narrow lead didn't come cheap.
The midsummer affair comes as Trump's shadow looms over primary contests in four other states on Tuesday, none bigger than Kansas, where the Republican president roiled the governor's race Monday by opposing the sitting GOP governor on the eve of the election.
For Republicans, it was costly: The Congressional Leadership Fund super PAC and the National Republican Congressional Committee, the House GOP's campaign arm, poured a combined total of more than $5 million into the race, compared to just $1 million from the Democratic Congressional Campaign Committee.
The races, like dozens before them, pit the strength of the Republican president's fiery coalition against the Democratic Party's anti-Trump resistance.
Outgoing Republican Gov. Rick Snyder had backed Calley, but Trump is more popular there with the GOP base in the state.
Balderson told supporters in OH: "I'd like to thank President Trump".
Republican strategists also conceded the race was tougher than initially envisioned.
Balderson said simply that it was an honor and "incredible" to appear with and be supported by the president.
President Donald Trump campaigned for Balderson, arguing Republicans need to control Congress and casting the midterms as a referendum on himself.
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Republican Troy Balderson may have narrowly eked out a win in Ohio's 12th congressional district over Danny O'Connor, but the victory came at a steep price. Pat Tiberi, was never even supposed to be in play. However, the OH secretary of state reported there are 8,483 outstanding absentee and provisional ballots left to count - much more than Balderson's 1,754-vote lead. "They can make it close, but they can't get over the hump", former Ohio Republican Party Chairman Matt Borges texted TPM as the final results rolled in Tuesday night. O'Connor had said he wouldn't vote for Pelosi as leader, but Republicans tried to link the two anyway. "We are in a tie ball-game", he told supporters at his election night party.
Voters are heading to the polls to decide key races including governor where seven Republicans and five Democrats are seeking their party's nomination.
Democratic challenger Danny O'Connor is critical of both Republicans and Democrats.
Whoever wins Tuesday will hold the seat until early 2019.
Voters in OH and Kansas join those across Missouri, Michigan and Washington state at the ballot box.
The Democratic race for governor in MI tested the electoral power of the party's progressive wing.
In the MI governor's race, progressive favorite Abdul El-Sayed, the 33-year-old former Detroit health official vying to become the nation's first Muslim governor, faces former state Senate Democratic leader Gretchen Whitmer, who is considered the favorite in the race.
They include John James, an African-American Republican, who won the Senate race nomination in MI.
On the Republican governor's ticket the top two contenders are Gov. Jeff Colyer and Secretary of State Kris Kobach. But only OH will send someone to Congress after votes are counted.
Trump made his preference clear for Kobach.