Toulouse's hopes of making the Top 14 knockout stages suffered a setback as they went down 9-3 to Oyonnax at Stade Charles Mathon on Friday.
Victory for the home side leaves them level on points with Toulouse in sixth and fifth spot provisionally ahead of the remaining weekend fixtures in the league.
However, Guy Noves' side will head home disappointed as they missed the chance to move above Racing Metro and instead leave the door open to the likes of Bordeaux-Begles, Grenoble and Montpellier to get amongst what is becoming a congested play-off race ahead of the final rounds in France.
A disappointing first-half on a poor pitch saw Oyonnax and a star-studded Toulouse outfit go in at the half-time break locked at three points apiece.
Boasting a side that included Louis Picamoles, Luke McAlister, Gaël Fickou and many more internationals, the visitors could not even trouble the scorers in the opening 36 minutes, with fly-half McAlister's boot finally levelling matters following Benjamin Urdapilleta's second-minute penalty for Oyonnax.
And the second-half unfortunately saw more of the same as no tries were scored in the fixture, with Urdapilleta landing further penalties in the 49th and 70th minute to seal a memorable triumph for Oyonnax, who move into the top six.
Pen: Urdapilleta 3
Oyonnax: 15 Florian Denos, 14 Silvere Tian, 13 Pierre Aguillon, 12 Roimata Hansell-Pune (c), 11 Yves Donguy, 10 Benjamin Urdapilleta, 9 Agustin Figuerola, 8 Viliami Ma'afu, 7 Olivier Missoup, 6 Valentin Ursache, 5 Damien Lagrange, 4 Leon Power, 3 Marc Clerc, 2 Jody Jenneker, 1 Antoine Tichit.
Replacements: 16 Paula Ngauamo, 17 Ruaan Du Preez, 18 Pedrie Wannenburg, 19 Maurie Fa'asavalu, 20 Fabien Cibray, 21 Regis Lespinas, 22 Hemani Paea, 23 Soane Tonga'uiha.
Toulouse: 15 Maxime Médard, 14 Vincent Clerc, 13 Florian Fritz, 12 Gaël Fickou, 11 Timoci Matanavou, 10 Luke McAlister, 9 Jean-Marc Doussain, 8 Gillian Galan, 7 Imanol Harinordoquy, 6 Louis Picamoles, 5 Romain Millo-Chluski, 4 Joe Tekori, 3 Census Johnston, 2 Corey Flynn, 1 Gurthrö Steenkamp.
Replacements: 16 Julien Marchand, 17 Schalk Ferreira, 18 Edwin Maka, 19 Yannick Nyanga, 20 Sébastien Bezy, 21 Clément Poitrenaud, 22 Yann David, 23 Neemia Tialata.
Referee: Romain Poite