The Argentine struck three times, while David Villa claimed a brace in a wonderful win for the Catalans. Cesc Fabregas and Xavi also scored, and Roversio put into his own net
Pep Guardiola's side had disappointling drawn their last two matches against Real Sociedad and AC Milan, having been pegged back after taking a two-goal lead against the Basque outfit and then conceding a last-gasp header to Thiago Silva in their Champions League opener on Tuesday.
Those 2-2 draws highlighted the need for a leader at the back in the absence of injured duo Gerard Pique and Carles Puyol, who had come on as a late substitute against the Italians.
Puyol was back in the side for his first appearance since April, having finally recovered from a knee injury, allowing Sergio Busquets - poor in each of the last two games as a makeshift centre-back - to return to his habitual role in central midfield.
Puyol began in a three-man back-line, alongside Javier Mascherano and Eric Abidal, with Dani Alves pushed forward in an advanced role as an extra midfielder.
And the new system paid dividends early on as both Busquets and Alves played a key part in Lionel Messi's fifth-minute opener.
Busquets picked out Alves' run down the right with a precise lofted pass and the Brazilian headed across the area for Messi, who stretched out his right foot to scramble the ball home at the far post.
Seven minutes later, Fabregas made it two after he was set up by Messi's lobbed pass and blasted a fine volley past Osasuna keeper Andres Fernandez.
Barca had been two up at Real Sociedad after 12 minutes last weekend, but ended up with only a point following the Basques' second-half comeback at Anoeta.
However, there appeared no chance of a repeat here as the Catalans remained rampant, with both Abidal and Messi heading against the woodwork before David Villa made it 3-0 after 33 minutes.
The Spain striker latched onto Abidal's clever through-ball, rounded Fernandez and slotted into the empty net to make it look so easy for the home side.
And it was, but Osasuna played their part in their own downfall as well, particularly as Barca went four up six minutes before the interval.
Abidal's long pass from his own half freed Villa, totally unmarked, and although the striker's shot was saved by Fernandez, Ruben could only direct the rebound into his own net.
Moments later, four then became five in what looked like a simple training ground exercise, with Xavi threading the ball to Cesc, and the former Arsenal captain turning to set up Messi, who slotted a low shot into the corner to give the hosts an unassailable lead at the break.
Messi then came desperately close to a hat-trick five minutes into the second half as he chipped a dlightful effort onto the crossbar, but the Argentine turned provider shortly afterwards as he curled a wonderful pass to Xavi to lob Fernandez for an exquisite sixth of the evening.
Puyol had already departed by then and he was followed off shortly afterwards by Xavi, as Guardiola sought to protect two of his leading lights.
Six goals to the good, Barca's intensity inevitably dropped and Nino came close at the other end, while Osasuna also saw an Ibrahima header ruled out for offside.
The home side had further, chances, though, and Villa curled a rasping effort just over the bar with 20 minutes left.
Six minutes later, Villa did score again as the Osasuna defence gifted the ball to Cesc in the area and the summer signing squared unselfishly for his team-mate to slot into the empty net.
Messi then made it eight as he completed his hat-trick, playing a clever one-two with the brilliant Fabregas and running through the Osasuna defence to slide a routine finish into the corner after some mesmerising build-up play around the visitors' area.
Osasuna will have been happy to have avoided double fugures in the end as they managed to negotiate the final 12 minutes withoug further damage.
The northerners, like Barca, had taken four points from their first two games and are now provisionally in eighth place.
Guardiola's side, meanwhile, move up to second, where they sit two points adrift of Valencia, but the Catalans could also be overtaken by Betis and title rivals Real Madrid, who face Levante on Sunday.
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