The New York Times gave Mr Balderson 50.2%, a lead of 1,754 votes in the 12th Congressional District race, with all precincts reporting.
Ohio Republicans argued that the race should be taken as a warning, but not a reason to panic.
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.
In June, a Monmouth University poll showed Balderson ahead by 10 points. He also called out Trump's brand of smash mouth politics - a move that many see as an attempt to court independents and anti-Trump Republicans.
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If you intend to vote in today's primary, you will need to either already be registered as a Republican or Democrat or declare a party affiliation at the polling place.
President Donald Trump is greeted by 12th Congressional District Republican candidate Troy Balderson as he arrives on Air Force One at John Glenn Columbus International Airport in Columbus, Ohio, Saturday, Aug. 4, 2018. "That was not good", the president tweeted.
Whoever wins Tuesday will hold the seat until early 2019.
Over in Kansas, Republicans are locked in an internal fight for the governorship. "Tomorrow we rest and we keep fighting through to November". The suburban-rural splits were huge, with O'Connor over-performing normal Democratic numbers near Columbus and Balderson racking up strong margins in the district's smaller towns and rural areas. At one point in the campaign, Balderson couldn't name a single area of disagreement but since cited differences on tariffs and immigration. Balderson and O'Connor will likely face off again in November at the mid-term elections.
His opponent's campaign was described by some analysts as "not serious".
President Donald Trump arrives in OH for rally
Fasano and House Minority Leader Themis Klarides are trying to turn the focus of CT citizens away from President Donald Trump . Hundreds of people are slowly making their way into an OH high school ahead of a rally headlined by President Donald Trump .
The pro-Balderson effort has focused largely on motivating Republican voters by casting O'Connor as extreme.
Three mainstream Democrats are viewed as leaders to vie for retiring Republican Rep. Dave Trott's seat in November.
Mr O'Connor and his Democratic allies have railed against the tax plan, casting it as a giveaway for the rich that exacerbates federal deficits and threatens Medicare and Social Security. The field includes Fayrouz Saad, who would be the first Muslim woman in Congress.
The Republican Party has held the OH 12th District seat for more than three decades, so the very fact that it's so close is significant.
But the Ocasio-Cortez and Bernie Sanders-backed Abdul El-Sayed's bid for MI governor fell short, CNN projects, as the establishment favorite, former state Senate Minority Leader Gretchen Whitmer, nabbed the Democratic nomination and a November date with Republican Attorney General Bill Schuette. "He will win BIG in Nov".
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Late this summer, Meghan is expected to return to the USA on a private plane to spend time with her mother and friends. The former American actress waved to well-wishers outside the church on Saturday, which is also her 37th birthday .