The deal was finalised on Tuesday and the Portugal striker, 23, will be eligible to play for the Premier League club from January.
Swansea manager Garry Monk wanted cover for frontman Wilfried Bony, who will join up with Ivory Coast for January's Africa Cup of Nations.
Oliveira has also been on loan at Pacos de Ferreira, Deportivo and Rennes.
The arrival of the 14-cap Oliveira gives Monk more options in his forward line, especially with second-choice striker Bafetimbi Gomis wanted by five other clubs.
"I'm really looking forward to it,'' Oliveira told the Swansea website.
"I already know quite a bit about Swansea and it was their style of play and the ambience at the club that attracted me. It was an easy decision to make.
"The deal was agreed early so I can spend an extra 10 days in Swansea training with the squad and getting to know my new team-mates. That way I hope to be ready to offer my services to the team that much earlier.
"I want to prepare properly for the challenge. It's going to be a big challenge because I have a lot of respect for fellow strikers Wilfried Bony and Bafetimbi Gomis.
"I have to earn the right and respect from the manager and my team-mates on the training ground to play. But I'm highly motivated to score goals and help the team.''
