Messi, who scored in the 18th, 63rd and 88th minutes, now has 371 career goals for Barcelona, two more than previous record holder Paulino Alcantara. Alexis Sanchez, Andres Iniesta, Cristian Tello and Pedro also scored for Barcelona, who bounced back from last weekend's surprising loss at Real Valladolid.
With the victory, third-placed Barca moved onto 66 points, four behind leaders Real Madrid and one behind Atletico Madrid in second.
Up ahead for Barcelona is next weekend's El Clasico against arch-rivals Real Madrid, a match that could prove pivotal in the Spanish title race. Victory for Real would open up a seven-point gap on Barcelona, but a win for Barcelona would cut their deficit to a single point. A draw could potentially benefit second-placed Atletico Madrid, who will enter the weekend three points behind Real.
Barcelona, meanwhile, remain alive in their quest for silverware in two more competitions. Having beaten Manchester City in the UEFA Champions League round of 16, Barca have advanced into the quarter-finals of that competition. In addition, Tata Martino's team will play rivals Real Madrid in the Copa del Rey final on April 16.
Perhaps sensing the need to keep pace with Real Madrid, Barcelona started well and led 3-0 at half-time. But Osasuna actually put the ball in Barcelona's net first, though Oriol Riera's ninth-minute tap-in was disallowed for offside.
Nine minutes later, Barcelona took the lead through Messi. Xavi freed Alexis on the right and Alexis squared for Messi, who tapped in to pull level with Alcantara on 369 career Barca goals.
Seven minutes later Messi had a chance to complete his hat-trick after a blistering Barcelona counter. Victor Valdes freed Pedro with a long clearance, allowing Pedro to tee up Messi with a pass to the back post. But Messi blasted his first-time shot over the bar with the goal gaping.
By then the match was beyond doubt, but Barcelona added a fifth goal three minutes later via Tello's superb curler. Messi finished off his hat-trick in the 88th minute, controlling Dani Alves' cross before slotting in to complete the scoring—and extend his record as Barca's all-time top scorer.
Pedro completed the rout in the 90th minute, finishing from Messi's pass to make it 7-0.
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