Olsen was struck by the difference between the high-tempo pressing game Japan played in a friendly against the Netherlands last year, when they lost 3-0 after dominating for the first hour, and the more cautious approach in their 1-0 defeat by the Dutch in their second Group E match in South Africa.
"They are a little more defensive, that is right," Olsen said ahead of his side's must-win meeting with the Japanese on Thursday evening. "They played with high pressure then and they play with low pressure now."
The switch has not affected Olsen's admiration for Japan's quality and he knows his injury-hit squad face a tough battle to overcome opponents who only need a draw to reach the last 16 of a FIFA World Cup for the first time on foreign soil.
"I said before the tournament that Japan was one of the teams that could surprise people who know nothing about Asian football," Olsen said. "They fight for each other and the team. You saw the Dutch team, with all the quality that they have, they had many problems against Japan. But my team can perform better than we have done so far and that is what we will need to do."
Asked to sum up his mood going into the match, Olsen replied: "I'm excited but not nervous. If you are not excited before a match like this you should stop as a coach. I trust my team but even if have a good day we can still lose. We hope for a lucky punch."
Olsen would not be drawn on whether Denmark's height advantage could make the difference, as has been widely predicted in the build-up to the match. Per Kroldrup, the Fiorentina defender who is expected to replace the suspended Simon Kjaer, was more forthcoming. "Yes, that's something we hope we can benefit from, that we are perhaps stronger than the Japanese in the air," he said.
Olsen has rubbished suggestions that his squad, one of the oldest in the tournament, has passed its peak. Pointing at the 32-year-old former Chelsea winger Jesper Gronkjaer, Olsen asked: "What's 32? He can still play for eight years. If a player is fit and not tired of playing it is fantastic.
"They have experience. They have played in European cups, World Cups, they know each other, as human beings as well as their strengths and weaknesses as players. They are quality players because they have been able to keep a high level for a long time."
Olsen has described Arsenal forward Nicklas Bendtner as doubtful for the match because of a groin injury which restricts his training to alternate days. It would be a major surprise however if Bendtner did not start a match in which Denmark will be aiming to maintain the country's proud record of always progressing beyond the first round when they make it to the FIFA World Cup finals.