A long-range strike from Cristiano Ronaldo left the home side with a one-goal lead at the interval.
Ronaldo scored his second in the early stages of the second period, before Sergio Ramos made it 3-0 on the hour mark.
Los Blancos were not finished, however, and made it four in the latter stages through Dani Carvajal.
Read how it all unfolded in Sports Mole's minute-by-minute live commentary of the action below.
REAL: Lopez; Nacho, Varane, Ramos, Marcelo; Illarra, Modric, Isco; Ronaldo, Morata, Di Maria
OSASUNA: Andres; Damia, Flano, Arribas, Oriol; Cejudo, Loe, Punal; Armenteros, Riera, De las Cuevas
LANDMARK: Cristiano Ronaldo has now scored 100 Liga goals in 81 Games at the Bernabéu.— Cristiano Ronaldo (@CristianoStats) April 26, 2014
20 - Real Madrid have scored more goals from outside the box than any other team in the top five European leagues this season. Cannon.— OptaJose (@OptaJose) April 26, 2014
Cristiano Ronaldo has now scored 30 league Goals (or more) for the 4th consecutive season.— Cristiano Ronaldo (@CristianoStats) April 26, 2014
29 - Ronaldo has scored more goals from outside the box than any other player in the top 5 European leagues since 09/10 (Messi 28). Missile.— OptaJose (@OptaJose) April 26, 2014