Denmark, who face the Portuguese on Wednesday, stunned the Netherlands 1-0 in the first game when they scored early and then were somewhat fortunate to hold out against their unusually wasteful opponents.
"We have to find better fitness, we have to be better than we were against Holland... if you win or lose there are always things you have to improve and there were some things we saw we did not feel good about," Olsen told a news conference.
"What makes this team strong is that they are mature, they think, they look themselves in the mirror and say 'Well, there were many things that were good but there are some things... that have to be corrected'."
Denmark have a good record against the Portuguese, losing just one of their last four competitive meetings. The Danes were in the same qualifying group as Portugal and finished up in first place.
One of the biggest threats is Portugal winger Cristiano Ronaldo, who was tightly marked by Michael Silberbauer when the two sides met in Copenhagen last October. The Danes won that game 2-1.
Olsen declined to reveal whether Silberbauer would carry out the same role again, saying merely that the player was fit.
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