The deal includes the option of making the transfer permanent at the end of the season, with the striker moving to the Stade de la Route de Lorient after spending last term with Deportivo La Coruna
The 21-year-old was not able help the Galician side avoid relegation from La Liga, netting just four goals in 31 appearances.
However, the Portuguese appears to have done enough to impress new Rennes boss Philippe Montanier, whose Real Sociedad side relegated Deportivo with a 1-0 win on the final day of last season.
"Nelson was our priority," Montanier told the club's official website.
"He is a promising and complete player."
Oliveira admitted that it was Montanier's presence that convinced him to join Rennes, and he says is hoping to repay the faith showed in him by the Britanny club.
"It (Montanier's influence) meant a lot. I observed how he evolved Real Sociedad last year," Oliveira explained.
"He called me several times and clearly explained to me that he wanted me. It was a very important element in me coming to Rennes.
"The most important thing will be to help Rennes. I want to repay the confidence that the coach and the club have shown in me.
"I hope to have a good season, score goals and do my best."