Oncins, who became a professional tennis player in 1988, initially only played at tournaments in his home country. In 1989 he won both his first single title and his first double title on the ATP Challenger Tour , and in the following year he won two more Challenger tournaments. In 1991 he won his first double title on the ATP World Tour at the side of Andrés Gómez in his hometown . He was also there in the individual finals, but lost to Jordi Arrese . He won his two ATP individual titles in 1992 at the tournaments in Bologna and Búzios . In total, he won five double titles in his career. He achieved his best ranking in the tennis world rankings in 1993 with position 34 in singles and in 2000 with position 22 in doubles.
His best individual result in a Grand Slam tournament was his entry into the second round of the French Open in 1992 . In the doubles competition, he made it to the semi-finals of the French Open twice. In 2001 he was in the mixed final of the French Open with Paola Suárez , where they were defeated by Virginia Ruano Pascual and Tomás Carbonell .
Oncins played 20 singles and 17 doubles games for the Brazilian Davis Cup team between 1991 and 2001 . His greatest success with the team was reaching the semi-finals of the world group 2000, where he was used in doubles. At the side of Gustavo Kuerten , he won his doubles against Nicolas Escudé and Cédric Pioline in the first round and against Dominik Hrbatý and Karol Kučera in the quarterfinals. In the 5-0 semi-final defeat against Australia they were defeated by Sandon Stolle and Mark Woodforde in three sets, each of which was decided in a tie-break .
At the 1992 Olympic Games he competed for Brazil in singles and doubles. He reached the quarter-finals, in which he was defeated Andrei Cherkassov . With his partner Luiz Mattar , however, he was eliminated in the first round. At the Olympic Games in Sydney he only appeared in doubles. Again, this time at Gustavo Kuerten's side, he didn't get past the first lap.
Brazil
Bologna
Luis-Enrique Herrera
Jordi Arrese
Christian Miniussi
Andrés Gómez
Casablanca
Marc-Kevin Goellner Eric Taino
Aleksandar Kitinov Jack Waite
Wellington
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Prague
Hendrik Jan Davids Libor Pimek
St. Polten
French Open