The French side ran in four first-half tries as Lucas Blanc, Metuisela Talebula, Clement Maynadier and Matthew Clarkin crossed at the Kassam Stadium.
Having trailed 31-6 at the break, there was some brief respite for a sparse home crowd when Seb Jewell managed to touch down, while Daniel Browne also got on the scoresheet.
However, Pierre Bernard scored another try for the visitors before Darly Domvo's double helped cap off an emphatic victory.
Bernard also bagged 15 points with his boot, while substitute Lionel Beauxis added another conversion, with Olly Barkley kicking over 10 points for London Welsh.