كتب : Mohamed Sabe
Oliveira inked a five-year contract with Milan for an undisclosed fee.
But the six-time European champions said in a statement that Swiss international midfielder Johann Vogel had joined Betis as part of the deal.
Milan have been in the market for a top-striker to replace Andriy Shevchenko who joined Chelsea earlier this summer.
The 26-year-old scored 30 goals for Betis since joining them from Valencia in 2004.
He was also a member of Valencia's squad that won the won the Primera Liga title and the UEFA Cup in 2004 under coach Rafael Benitez.