The Champions League holders recorded a 4-0 home win in the first leg, but Guardiola has made it clear Osasuna should not be under-estimated.
"The Copa del Rey is a very special tournament. There are 90 minutes left [in this tie] and Osasuna have their virtues, which we already know about. It's our duty to realise that we still have work to do," Guardiola told Barcelona's website.
"All grounds are complicated, and this one especially, for its size and the way they play. We know it will be cold, and we'll see how the pitch is. We'll try and play well to reach the quarters."
Guardiola also insisted his players will not be approaching the match any differently and will still aim to dominate and win the game.
"We hold a good lead, but we can't rely on that," he said. "We'll try to win the game. I don't know any other way. We'll try and play well and control the pitch, and the result. We'll go out to play the best we can, which is the only way I know."
The winner of the tie will face Real Madrid in the quarter-finals.
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