A goal from Joseba Llorente has left Osasuna 45 minutes away from recording a victory over Barcelona at the El Sadar.
Pre-season favourites to win the La Liga title Barcelona started brightly in Pamplona, creating numerous chances during the early exchanges.
The best fell to Alexis Sanchez, who drove his shot straight at Andres Fernandez in the home goal with eight minutes on the clock.
Hosting Osasuna took a while to settle into the encounter, but when they eventually did, they went in front. Roland Lamah drilled in a cross for Llorente, who turned the ball past Victor Valdes in the 17th minute.
Llorente had the chance to double his side's advantage with 30 minutes played, but Valdes reacted quickly to deny the attacker.
As the half drew to a close, Andres Iniesta saw his low effort held by Fernandez, while Cristian Tello struck the post.
