The pair won the men's final to earn their country's first gold medal since Katie Taylor's boxing victory in 2012.
They also became the first male Irish gold medallists since boxer Michael Carruth in 1992 after beating Germany by just 0.86 seconds at the Sea Forest Waterway on Thursday.
It was O'Donovan's second Olympic medal after he won silver in the double sculls with brother Gary in Rio. He becomes the first Irish athlete to win Olympic gold and silver medals.
He said: "I didn't think about too much how I felt at the time, so it's difficult to compare it from five years after.
"Winning today and silver medal last time, I'll be a bit happier about that. The silver medal is nice but Fintan did the right job and we went straight to the top at his first Games.
"The two boys (Germany) have been looking strong all week and made us really work hard. Italy were quick off the start and didn't let us have it easy."
The duo had set a new Olympic best in the semi-final of six minutes 05.33 seconds but had to fight for gold.
Germans Jonathan Rommelmann and Jason Osborne were 10 lengths clear after 500m but Ireland slowly closed the gap and managed to edge ahead and were a length in front before the finish line.