Mr Trump took to Twitter before the final vote was confirmed, tweeting: "Despite major outside money, FAKE media support and eleven Republican candidates, BIG "R" win with runoff in Georgia". He will compete in a June runoff against former Georgia secretary of state Karen Handel.
While Ossoff and national Democrats acknowledged that they hoped to win the seat outright Tuesday due to the crowded and fractured GOP field, Ossoff will likely enter the runoff with Handel as a slight favorite.
"It's a test case for how far a Democratic candidate can go in a Republican district on the strength of opposition to Trump", said Todd Rehm, a Georgia Republican strategist who is not affiliated with any candidate in the race.
The seat Ossoff nearly won was represented by conservative firebrand Newt Gingrich for decades and more recently by tea party congressman Tom Price, now the health and human services secretary.
"Dems failed in Kansas and are now failing in Georgia", Trump tweeted, referring to last week's congressional race in a Wichita suburb where the Democrat fell short of an upset.
Republicans barely avoided an embarrassing setback by forcing a runoff election for a historically Republican open House seat Tuesday night - and that's cause for celebration in the eyes of President Trump.
It's strongly supported Republican presidential candidates, too, giving Mitt Romney a greater than 20-point win in 2012. "Glad to be of help!"
"There is no doubt that this is already a victory for the ages", Ossoff told his ecstatic supporters Tuesday night. The suburban Atlanta district has been represented by a Republican for more than two decades.
Now that the choice is directly between Handel and Ossoff, Trump could get more directly involved in rallying district voters with a trip to Georgia. Newt Gingrich came from this district.
Democrat sachems cleared the field for 30-year-old Jon Ossoff and showered him with millions of dollars.
The president weighed in on the high-stakes election earlier Tuesday, predicting "Ossoff would be a disaster in Congress".
Republicans say they could beat Ossoff in a one-on-one contest. As of press time, just after 11 p.m., Cobb and DeKalb had reported their election numbers to the secretary of state's office, but Fulton County was lagging behind, with only 17 percent of its precincts in.
And they came close.
"One that they backed, Spicer said, "and I think when you look at the total Republican vote, it was over that".
The Democrat lost that race by seven percentage points, but the party sought to spin the result as a positive, pointing out that Trump won the district in November by 27 points.