The former Wolves and Bournemouth boss was drafted in as Rosenior’s replacement just three days ago.
But O’Neil immediately upstaged his predecessor by guiding Strasbourg to a 6-0 win in the French Cup over Avranches.
Strasbourg’s new gaffer turned to ex-Premier League stars including Ben Chilwell, Julio Enciso and Valentin Barco as his side strutted to victory.
And O’Neil immediately bettered Rosenior, who never saw Strasbourg score six goals during his 18 months at the club.
The new Chelsea manager’s biggest score came in a 5-0 win against Angers in October.
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But Rosenior had plenty to smile about on Saturday too as his first Blues match ended in a 5-1 FA Cup win against Charlton.
As for O’Neil, he reflected on a “crazy” start to his new life in France, hailing his players for their professionalism at the shock change in management and subsequent response on the pitch.
O’Neil said: “It’s been a crazy three days. Obviously it moved very, very fast, but today it was important that we got the job done.
“The scoreline 6-0 makes it look easy but it’s only easy because we did things properly from the moment I came training on Thursday and training yesterday.
“The way the boys approached the game today, very professional and a good win for everyone.”
O’Neil’s next focus will be leading Strasbourg up the Ligue 1 table.
His new team sit seventh in the table with seven wins from 17 outings.
But they face a tough run, taking on France’s top five sides Lens, PSG, Marseille, Lille and Lyon in their next seven games.