Goals from Cristhian Stuani and Portu helped overturn Isco's early goal and inflict the second defeat of the season on Zinedine Zidane's men.
The result was also very welcome in Barcelona, as it confirmed an eight-point lead for the Blaugrana over Madrid with 10 games played in the Liga season. Barca have got the better of their Clasico rivals many times over the last few years, but their luck has not been shared by other teams in the region.
Indeed, Sunday's upset was the first time a Catalan side other than Barca had beaten Madrid in La Liga for more than 10 years, with Espanyol having been the last team to get the better of them way back in October 2007.
And to make matters worse for Madrid, their record-breaking away run in Spain's top flight was finally brought to an end.
Before the game at Estadi Montilivi, Zidane's side had won their last 13 away matches in a row in La Liga - a record unmatched by any other side in the competition's history.
Olunga will now have to wait for another start on Sunday (November 5) against Levante.