Bale came off the bench against Celta Vigo last weekend but this was his first start since fully recovering from a calf strain.
Bale and Cristiano Ronaldo started for Real Madrid in what was a strong selection by Ancelotti, and indeed Los Blancos started the brighter of the two sides.
Luka Modric and Benzema both had sights of goal in the opening exchanges, with Osasuna showing some adventure on the counter attack.
Real Madrid thought they had won a penalty kick when Perez Montero pointed to the spot after Asier Riesgo had brought down Jese Rodriguez in the box. But once the referee realized his mistake, with Riesgo getting a solid hand to the ball, he decided to book Jese for diving instead.
But the home side did take the lead just seconds later, when Benzema rose to meet Luka Modric’s in-swinging freekick with a glancing header into the Osasuna net.
Ronaldo was enduring a quiet game by his usual high standards but came close to finding the net, with Alejandro Arribas forced to make a last ditch sliding tackle to prevent the Portuguese winger from tapping home Jese’s squared pass across the face of goal.
Despite Real Madrid’s dominance Osasuna had the chance to restore parity when Oier Sanjurjo fluffed his shot 10 yards out after taking an unmarked position inside the opposition box.
The second half saw the hosts took full control of the contest, with Benzema passing up a glorious chance to add his and Real’s second of the night, slicing a shot well wide of the target from a good position yards from goal.
However, Real Madrid finally notched the next goal, with Jese Rodriguez taking advantage of some slack defensive play by Osasuna to calmly finish high into the net.
Ronaldo was then afforded two clear cut chances to put the tie to bed before the return leg but firstly he volleyed a shot straight at Riesgo after finding himself unmarked at the back post, before secondly flicking a Modric pass over the top inches past the post.
The home side continued to flood forward in search of a third, with Ronaldo once again looking most potent. But this time he headed over at the back post, accidentally clattering teammate Benzema in the process.
And Ronaldo was centre of attention minutes later, getting in the way of Bale’s goalbound shot from six yards to deflect the ball wide of the target, when a goal looked inevitable.
Alvaro Morata was introduced in the final few minutes as Ancelotti’s made a final throw of the dice in search of a third goal but Real Madrid were made to settle for a 2-0 win to take into the second leg.