The Lisbon club ended the group phase above second-place Basel who shocked Premier League champions Manchester United with a 2-1 win in the final match.
"We must stress this brilliant campaign of ours," Benfica coach Jorge Jesus told reporters. "Benfica's group had three excellent teams and United, who were the favorites, were knocked out, that says it all."
Argentine winger Nicolas Gaitan ran to the byline inside Otelul's box and his clever pass found the Paraguayan striker who coolly tapped in the winner in the seventh minute.
"We are very ambitious we accomplished the objective of being first but the key goal was to get to the last 16 round. Now we will see what the draw has in stall for us and in what form we will be in the knockout," Aimar said.
Cardozo, Benfica's top goalscorer in the competition with three goals, almost grabbed a second in the 33rd minute with a shot from outside the box that went inches wide.
Moments later, in a rare chance for the Romanians, forward Gabriel Paraschiv caught Benfica's defence napping but lost out in the one-on-one duel with Benfica keeper Artur, who also saved Gabriel Giurgiu's rebound shot.
Benfica kept control in the second half and Otelul took no risks. Cardozo headed over keeper Branko Grahovac's goal, Javi Garcia went close with a cracking shot from distance and substitute Rodrigo also missed two good chances.
"It was important to finish first because it allows us to escape from meeting a few big sharks in the next round," Benfica's Ruben Amorim said.
