The TV spot – part of a seven-figure weekly buy, set to air on Tuesday – paints the candidate as an advocate for voting rights who will “listen to the experts” to combat the coronavirus pandemic.
"He learned about public service from one of my lifelong heroes, John Lewis," Mr Obama says in the ad, invoking the late Civil Rights activist and Georgia congressman whose life’s work sought to protect the Voting Rights Act and enfranchise Americans.
“He knows we need a new Voting Rights Act that makes sure every Georgian is treated equally under the law,” Mr Obama says.
He also says that Mr Ossoff will “listen to the experts to get this pandemic under control.”
"If we vote like our lives depend on it, because they do, we will elect Jon Osoff to the United States Senate," Mr Obama says.
Mr Ossoff faces incumbent Republican Senator David Perdue, who has deflected attacks from his opponent and prominent Democrats over his multi-million dollar stock trades during the Covid-19 crisis, which have drawn scrutiny from the Department of Justice and Securities and Exchange Commission.
The two remaining US Senate runoff elections are both in Georgia, and winners in those races could determine whether the body is flipped from GOP dominance to Democratic control, shifting the power balance in Congress.
Democratic candidate Raphael Warnock, a senior pastor at Atlanta’s Ebenezer Baptist Church, is challenging Republican Senator Kelly Loeffler.
More than 940,000 voters have requested mail-in ballots for the runoff election on 5 January, according to Georgia elections officials.
In the November general election, more than 1.3 million absentee ballots were cast, Georgia’s secretary of state announced.
Neither Mr Ossoff or Senator Perdue received more than 50 per cent of the vote in November, with the senator eking out more 86,000 more votes ahead of his Democratic rival.
Democrats, including voting rights advocate and former Georgia gubernatorial candidate Stacey Abrams, have focussed on mobilising Black, Latino and young voters to ensure Democratic victories, as the campaigns attract national attention among Democratic organisers pushing “get out the vote” drives.
The deadline to register to vote in time for the runoff election is 7 December.