After declaring he was in "perfect condition" in the build-up to Saturday's match, Cristiano Ronaldo proved it on the pitch with a dazzling display and two goals, before being taken off in preparation for Tuesday's second leg with Bayern.
His sixth-minute strike gave Andres Fernandez no chance as it flew into the back of the net from long range, and he created a handful of chances for his team-mates that were wasted.
Osasuna rarely threatened Diego Lopez and they were unable to bring the goalkeeper into action when they did have a sight of goal.
Dani Carvajal scored a late header to underline Real's dominance and keep the heat on Atletico Madrid and Barcelona in the title race, the pair facing respective trips to Valencia and Villarreal in Sunday's action.
With one eye on Tuesday's trip to the Allianz Arena, which Real will enter 1-0 ahead, Carlo Ancelotti made six changes to his starting line-up but it was a man who featured in their semi-final first leg that opened the scoring.
Having wasted chances in the first half, Ramos finally got his name on the scoresheet as he charged into the penalty area to meet Angel di Maria's cross.
One of the substitutes made an immediate impact as Carvajal, who had replaced Di Maria, escaped the attentions of Osasuna's defenders and lobbed a header over Andres from Isco's chipped pass.
