Diego Costa grabbed a double as the capital club made it six wins from six to sit just behind Barcelona on goal difference.
First, he opened the scoring with 18 minutes on the clock as he fired a Juanfran cross past Andres Fernandez with precision; then the same player doubled the advantage only six minutes later when he rose highest to head a Jorge Koke cross past the keeper.
While Osasuna cut the arrears with 42 minutes gone through a Oriol Riera strike, the result was never really in doubt as the hosts saw the second half out with relative ease.
In contrast, Almeria are still without a win as they were defeated by Malaga 2-0 at La Rosaleda.
Fernando Tissone and Francisco Portillo got the goals as Malaga’s recovery from a sluggish start begins to gather pace.
Despite being the better side, Bernd Schuster’s men had to wait until the 69th minute to open the scoring, Tissone reacting quickest to a Sergio Sanchez glancing header to poke home at a corner.
Second-half substitute Roque Santa Cruz then provided the assist as Portillo curled home to secure the win that leaves Malaga in seventh place.
Ten-man Real Valladolid held Levante to a 1-1 draw at Estadi Ciutat de Valencia.
Papa Baba Diawara gave the home side the lead on 36 minutes when he swept home from close range but their lead only lasted six minutes as Javi Guerra levelled things up.
Valladolid's left-back Zakarya Bergdich was given his marching orders in the 44th minute for a foul on Jordi Xumetra inside the box but Andreas Ivanschitz’s penalty was saved by Diego Marino.
Despite their numerical advantage, Levante could not force the winner and were forced to settle for a share of the spoils.