Castres have made two more major signings ahead of next season with the arrivals of Benjamin Urdapilleta and Antoine Tichit.
Urdapilleta arrives on a two-year deal after finishing last season as the second-highest points scorer in the Top 14 with 297.
The Argentinian fly-half has been a regular matchwinner for Oyonnax in his time at the club, helping them to a sixth-placed finish this season.
Prop forward Tichit meanwhile also arrives at the 2013 champions on a two-year deal from Oyonnax, as Castres look to rebuild following their narrow escape from relegation.
Urdapilleta will replace the outgoing Rémi Talès, who announced that he would join Racing Métro earlier this week.
Castres have also signed Toulon wingers David Smith and Rudi Wulf, along with Alex Tulou and Alexandre Bias from Montpellier.