Jordan Loties of France, second right, celebrates after his team scored the second goal against Real Madrid, during their Spanish League soccer match, at El Sadar stadium, in Pamplona northern Spain on Saturday, Dec. 14, 2013. (Photo: AP)
Carlo Ancelotti's side began confidently having won their last five league games, scoring 24 goals in the process, but were undone by two first half headers from Oriol Riera.
Sergio Ramos, returning from suspension and whose discipline has been questioned, was red carded after 43 minutes but Isco gave Real hope with a strike on the point of halftime.
Bale was taken off 10 minutes into the second half as they searched for an equaliser that eventually came from a Pepe header after Osasuna were also reduced to 10 men with Francisco Silva dismissed.
Bale, a 100 million euros signing, scored a hat-trick in his last league game against Valladolid but was unable to make an impact against Osasuna who also managed to earn a draw against Barcelona earlier in the season.
Later Barcelona can go three points clear at the top if they beat Villarreal with Atletico Madrid, level on 40 points playing on Sunday. Real are third with 38 points.
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