The Blues have confirmed the acquisiton of the 20-year-old International midfielder, who had also interested Tottenham and is currently representing Brazil at the Olympic Games.
The Blues confirmed the deal on their official website on Wednesday having verbally agreed the transfer, according to the Brazilian club’s president, Giovanni Luigi, earlier in the week.
No details have been released regarding the player’s transfer fee or contract, but reports had claimed that the playmaker would cost around £25 million.
Oscar is currently representing Brazil at the Olympic Games and played in the Selecao’s pre-tournament warm-up friendly against Great Britain.
He is Chelsea’s fourth signing of the summer, joining Marko Marin and brothers Eden and Thorgan Hazard at Stamford Bridge.
Oscar joined Internacional from Sao Paulo in 2010 and has won the Campeonato Gaucho state championship twice with the Porto Alegre-based side.
He scored 13 goals in 44 matches in all competitions last season and has found the net six times in 20 appearances during the current campaign.