Biarritz had Pelu Taele sent-off as they fell to their seventh defeat of the season after losing the Basque derby 27-19 against Bayonne on Saturday.
The second-row's dismissal came in the ninth minute and left BO with a mountain to climb.
Elsewhere in the Top 14 this Saturday, Castres edged out Racing MÃ©tro 19-15 while Brive fought back to claim a decent 12-12 draw at Grenoble.
Perpignan beat Toulouse 20-16 in the late game.
Stade FranÃ§ais 18-11 Montpellier
Oyonnax 25-22 Toulon
Bayonne 27-19 Biarritz
Castres 19-15 Racing MÃ©tro
Grenoble 12-12 Brive
Perpignan 20-16 Toulouse
Sunday, September 28
Clermont v Bordeaux-BÃ¨gles
Aviron Bayonnais 27-19 Biarritz Olympique
Stade Jean Dauger, Bayonne
Biarritz Olympique are now nine points adrift at the bottom of the Top 14 standings after losing the Basque derby to fellow league strugglers Bayonne at Stade Jean Dauger.
Form has not been either side's friend of late but when BO second-row forward Pelu Taele was shown a red card as early as the ninth minute, the visitors' fate was all but sealed.
Bayonne did not find life easy however and could only cross the whitewash twice, through Marvin O'Connor and Mike Phillips, with Martin Bustos Moyano kicking seventeen points.
Joe Pietersen was Biarritz's lone scorer as he got a try, conversion and four penalties ahead of the Top 14 break for Heineken Cup action, which will be an opportunity for BO to regroup.
Tries: O'Connor, Phillips
Pen: Bustos-Moyano 5
Pen: Pietersen 4
Bayonne: 15 Scott Spedding, 14 Martin Bustos Moyano, 13 Joe Rokocoko, 12 Manu Ahotaeiloa, 11 Marvin O'Connor, 10 Stephen Brett, 9 Mike Phillips, 8 Opeti Fonua, 7 Julien Puricelli, 6 Dwayne Haare, 5 Lisiate Faaoso, 4 Abdellatif Boutaty, 3 Nemiah Tialata, 2 David Roumieu, 1 JC Janse van Rensburg.
Replacements: 16 Gregory Arganese, 17 Gert Muller, 18 Mark Chisholm, 19 Jean Monribot, 20 Sam Gerber, 21 Matthieu Ugalde, 22 Guillaume Rouet, 23 Vitolio Manukula.
Biarritz: 15 Paul Couet Lannes, 14 Takudzwa Ngwenya, 13 Seremaia Burotu, 12 Damien Traille, 11 Joe Pietersen, 10 Daniel Waenga, 9 Luix Roussarie, 8 Ueleni Fono, 7 Tanguy Molcard, 6 Joshua Furno, 5 Pelu Taele, 4 Erik Lund, 3 Francisco Gomez Kodela, 2 Jean Philippe Genevois, 1 Alexandre Menini.
Replacements: 16 Romain Ruffenach, 17 Thomas Synaeghel, 18 Addison Lockley, 19 Imanol Harinordoquy, 20 Yann Lesgourgues, 21 Benoit Baby, 22 Julien Peyrelongue, 23 Eugene van Staden.
Castres Olympique 19-15 Racing MÃ©tro 92
Stade Pierre Antoine, Castres
Castres responded from last week's heavy loss at Toulouse by scoring a narrow win over Racing, a victory that takes them up to sixth ahead of the week's two remaining games.
Marcel Garvey got the game's only try, which came on the half-hour mark, with Castres leading 13-9 at the break despite Racing having opened matters via Benjamin Lapeyre.
South African Rory Kockott kicked eleven points while Remi Tales' own drop-goal kept Jonathan Sexton and the Paris side trailing against the reigning Top 14 champions.
Pen: Kockott 3
For Racing MÃ©tro:
Pens: Sexton 4
Castres: 15 Brice Dulin, 14 Romain Martial, 13 Romain Cabannes, 12 Remi Lamerat, 11 Marcel Garvey, 10 Remi Tales, 9 Rory Kockott, 8 Antonie Claassen, 7 Yannick Caballero, 6 Jan de bruin Bornman, 5 Richie Gray, 4 Christophe Samson, 3 Anton Peikrishvili, 2 Brice Mach, 1 Yannick Forestier.
Replacements: 16 Mathieu Bonello, 17 Mihaita Lazar, 18 Benjamin Desroche, 19 Pedrie Wannenburg, 20 Julien Tomas, 21 Seremaia Bai, 22 Paul Bonnefond, 23 Michael Coetzee.
Racing MÃ©tro: 15 Benjamin Lapeyre, 14 Virimi Vakatawa, 13 Henry Chavancy, 12 Alexandre Dumoulin, 11 Marc Andreu, 10 Jonathan Sexton, 9 Maxime Machenaud, 8 Jacques Cronje, 7 Antoine Battut, 6 Wenceslas Lauret, 5 Francois van der Merwe, 4 Karim Ghezal, 3 Luc Ducalcon, 2 Virgile Lacombe, 1 Davit Khinchagishvili.
Replacements: 16 Dimitri Szarzewski, 17 Eddy Ben Arous, 18 Jone Qovu, 19 Fabrice Metz, 20 Bernard le Roux, 21 Sebastien Descons, 22 Fabrice Estebanez, 23 Brian Mujati.
FC Grenoble 12-12 CA Brive-CorrÃ¨ze
Stade LesdiguiÃ¨res, Grenoble
A 72nd minute penalty from Brive full-back Gaetan Germain denied Grenoble all four points.
The hosts had led until Germain's late effort, which will make the draw difficult to swallow.
Valentin Courrent was Grenoble's points contributor as he slotted over four penalties to Germain's four at Stade LesdiguiÃ¨res in a result that leaves the pair outside the top six.
Pen: Courrent 4
Pens: Germain 4
Grenoble: 15 Fabien Gengenbacher, 14 Benjamin Thiery, 13 Rudolf Coetzee, 12 Geoffroy Messina, 11 Julien Caminati, 10 Blair Stewart, 9 Valentin Courrent, 8 Florian Faure, 7 Jonathan Best, 6 Roland Bernard, 5 Andrew Farley, 4 Hendrik Roodt, 3 Dayna Edwards, 2 Vincent Campo Castillo, 1 Kenan Mutapcic.
Replacements: 16 Romain David, 17 Anthony Hegarty, 18 Naude Beukes, 19 Cedric Beal, 20 Benjamin Hand, 21 Ratu Ratini, 22 James Hart, 23 Albertus Buckle.
Brive: 15 Gaetan Germain, 14 Guillaume Namy, 13 Arnaud Mignardi, 12 Thomas Laranjeira, 11 Alfi Mafi, 10 Riaan Swanepoel, 9 Thomas Sanchou, 8 Sisa Koyamaibole, 7 Tjiuee Uanivi, 6 Said Hireche, 5 Olivier Caisso, 4 Julien Ledevedec, 3 Tamato Leupolu, 2 Guillaume Ribes, 1 Karlen Asieshvili.
Replacements: 16 Francois da Ros, 17 Johannes Coetzee, 18 Arnaud Mela, 19 Kieran Murphy, 20 Damien Neveu, 21 Romain Sola, 22 Andrew Mailei, 23 Kevin Buys.
USA Perpignan 20-16 Stade Toulousain
Stade AimÃ© Giral, Perpignan
Perpignan beat a weakened Toulouse line-up in Saturday's late game in a result that propels them up the standings and leaves their visitors sitting in fourth position.
James Hook was the hero as his fifteen points - three of which coming from a 50 metre drop-goal - guided USAP to victory. Sofiane Guitoune had opened matters with an early try.
Toulouse, who did not start Luke McAlister, Hosea Gear, Christopher Tolofua and Census Johnston, troubled the scorers through Sebastien Bezy's try and five points from the tee.