Oscar joins Sao Paulo after spending eight years in China with Shanghai Port.
The veteran has signed a three-year deal with Sao Paulo.
"I'm happy to be back in Brazil and to be able to play for Sao Paulo, which is the club where I started out, where I made my base and where I grew up," the 33-year-old said in a statement.
"I thank you for the affection I have received on social media these past few days and I will do my best to achieve great things together."
Sao Paulo president Julio Casares also said: "Today officially marks the end of a chapter that has profoundly influenced my life.
"After eight years wearing this shirt, it is difficult to describe the complex emotions I feel as I say goodbye.
"These were intense years, filled with hard work, dedication, and, above all, victories. Together, we won everything possible, and this was only achieved thanks to the unconditional support of everyone who is part of this club and its fans. I want to thank the club for believing in me from day one.
"The trust, the support, and the conditions you always provided were fundamental for my growth as a player and as a person. I take with me incredible memories and the certainty that I gave my all for this club."
Oscar left Chelsea for Shanghai Port in 2016 for £60m.