The former Manchester United striker had his hat-trick before the break as Fabio Capello's side moved to with two points of current leaders Sevilla who were held 0-0 at Racing Santander.
Dubbed 'Van Gol' by the Spanish press, van Nistelrooy was in vintage form as he took his goal tally for the Madrid to 13 in the last 15 matches.
His form is a huge relief to a side which has just learnt it will be without Brazilian striker Ronaldo for another month due to the portly forward’s knee injury.
The former PSV striker also joined the top ranks of the league goal scoring table, as his eight in La Liga put him alongside Diego Milito and Frederic Kanoute.
Osasuna could also find themselves in trouble with authorities after a fan threw a cigarette lighter at Spanish goal keeper Iker Casillas, although he was not seriously injured.
Mali striker Kanoute was unable to add to his tally at Racing Santander whose impressive defence has conceded just 10 goals all season.
On Saturday, crisis-hit Valencia conceded a last-minute equaliser as they drew 1-1 with struggling Athletic Bilbao.
Valencia have taken just two points from their last three matches with the club now lying in sixth place in the table, four points behind Sevilla.
Saturday's draw came at the end of a week when coach Quique Flores and sporting director Amedeo Carboni fell out while star striker David Villa also had a training ground bust-up with Flores.
Earlier Saturday, Atletico Madrid beat Villarreal 3-1.