The Greek champions opened their Champions League campaign in stunning fashion, upstaging 2013-14 finalists Atletico 3-2 in Group A action at the Karaiskakis Stadium.
Olympiacos were arguably second best throughout but made the most of their opportunities, with goals from Arthur Masuaku and loanee Ibrahim Afellay helping the hosts to a two-goal lead inside 31 minutes.
Mario Mandzukic pulled a goal back for Atletico before half-time but Greece international Kostas Mitroglou restored Olympiacos' two-goal cushion after the break and that proved enough, despite Antoine Griezmann's late strike.
Speaking post-game, Michel said the La Liga champions were caught off guard by his team's tactics that mirrored their opponent's.
"We forced Atletico out of their comfort zone, which is what they usually do to their opponents," said the 51-year-old, who guided Olympiacos to the quarter-finals last season.
"We played like a team, like [Diego] Simeone's side does. All of my players understood what they had to do. We are very happy. We tried to spoil their game and we succeeded.
"Atletico haven't conceded three goals [in a game] in a very long time."
Michel added: "All coaches study their opponents closely, but today my players worked incredibly hard. We have got the win and we are delighted.
"We played a bit like Atletico usually do and we took them by surprise. It worked out very well for us and we got our tactics spot on.
"You don't achieve things like we achieved tonight every day. Getting into the last 16 again would be great news for us – it would even better than when we did it last season."
