Ravel Morrison has returned to parent club West Ham from Cardiff City on compassionate leave.
Brighton boss Sami Hyypia is weighing up a move for former France captain Alou Diarra.
The clubs are believed to have agreed a fee of around 16million euros (£12.8m) for the Slovenia international, who will replace Thibaut Courtois as the Primera Division champions' first choice keeper.
The Belgian has returned to Chelsea after spending three years on loan at the Vicente Calderon.
Oblak revealed he is to swap the Portuguese capital for Madrid in an interview with newspaper A Bola.
"It was my decision to leave for Atletico," said the 21-year-old.
"Ever since I was a child I have dreamed about playing in the Spanish league, which is a better league than the Portuguese one. This was always a big challenge that I wanted to take on.
"Benfica wanted me to stay and the president has done everything to stop me from leaving, but my wish was to sign for Atletico."
The keeper also admitted he will receive a pay rise by moving to Atletico.
He said: "The financial factor was not the main reason, but the conditions are much better."
Oblak signed for Benfica in 2010 from Slovenian side Olimpija and spent his first three years in Portugal out on loan before becoming the Lisbon side's number one keeper last season.
He won a treble of the Portuguese league title, domestic and league cups, but played as Benfica lost the Europa League final to Sevilla on penalties.
"I will never forget the years I had with Benfica," he added.
"I wish everyone at the club all the best and I'm grateful for the help I received there. Benfica is a fantastic club."