Olivier was the match referee for Ash and Paul's battle at Lake Acuity. Paul's brother, Reggie, asked him to serve as the judge for the Pokémon Battle. Since Team Rocket ripped up the Pokemon Center's battlefield, he agreed to referee the match on an open plain next to Lake Acuity.
Olivier was unaware that Team Rocket was behind the 'repairs' of the Pokémon Center's battlefield and that they were his assistants for the match due to their clever disguises.