Earlier, Sevilla leapfrogged Valencia to take second place after spectacular strikes from defender Aitor Ocio and striker Julio Baptista earned a 2-1 victory at home to Atletico Madrid.
Barca, the only unbeaten side in the league, lead the table with 22 points from eight games, Sevilla have 17 and champions Valencia are third, a further three points back after slipping to their second successive defeat.
Real Madrid recorded their first league win in four games on Saturday when they beat Valencia 1-0 at the Bernabeu thanks to a classy strike from England striker Michael Owen.
The victory left Real seventh in the standings, nine points behind arch-rivals Barcelona.
Osasuna, who had not lost on their last three visits to the Nou Camp, provided stiff resistance to Barcelona in the first half despite missing injured striker Savo Milosevic and suspended midfield enforcer Pablo Garcia.
But the Catalans never lost their patience and were rewarded when they grabbed two quick-fire goals just before the break.
Ronaldinho chipped a clever pass over the defence to towards striker Eto'o on the left of the area and the African Player of the Year caught keeper Ricardo Sanzol off guard when he volleyed into the far corner.
The Navarrans hardly had time to draw breath before they went 2-0 down, Ronaldinho scoring from the penalty spot after he was adjudged to have been pushed by midfielder Patxi Punal as he jinked into the area.
The game lost momentum after the break, but Barcelona wrapped up an emphatic win in stoppage time when Eto'o danced into the area, exchanged a pass with midfielder Andres Iniesta and drilled home his seventh goal of the season.
Joaquin Caparros's young Sevilla side made it four home wins out of four as they outplayed visitors Atletico with their powerful mixture of intense and skilful football.
THUNDERBOLT SHOT
Ocio put them ahead after 11 minutes when he unleashed a thunderbolt from just outside the area after Atletico midfielder Jorge had conceded a free kick.
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Atletico gained a late consolation when Ibanez smashed home from close range after a goalmouth scramble in stoppage time.
The performance of the day came from Argentine midfielder Juan Roman Riquelme, who scored one goal and made two to inspire Villarreal to a 4-0 victory over struggling Numancia, who dropped to the bottom of the table.
Villarreal's second win of the season helped them recover from a difficult week in which Brazilian midfielder Marcos Senna was suspended after a positive drug test and fellow countryman Sonny Anderson left to play in Qatar.