The 17-year-old replaced Fabio Abiuso on 79 minutes as the title head to extra-time following a 1-1 drawn game in regulation time.
After a feisty but goalless first half, the Giallorossi took the lead in the 70th minute when Javier Vicario rose highest to head past William Rovida.
Inter struck back ten minutes later when Franco Carboni’s inch-perfect delivery set up Cesare Casadei for a headed equaliser.
As the game went to extra time, substitute Nikola Iliev came to the fore. Having previously hit the woodwork, he linked up superbly with Jan Zuberek before firing home inside the near post to complete the comeback.
Owusu played 13 times in the league for the side.
He has been capped twice at the U17 level by Italy.