The world's most expensive player rescued his team, heading in Gonzalo Higuain's cross from a metre out, for his second of the night and 22nd of the campaign.
Real made life difficult for themselves against mid-table visitors twice going behind to goals from Carlos Aranda and Krisztian Vadocz. Ronaldo and Marcelo grabbed the equalisers before the break.
"It seemed like an easy game on paper but it turned out to be tough. This often happens in these types of games. You have to show plenty of fighting spirit to win," Ronaldo told Spanish television.
"The win was the most important thing because we still have hopes of beating Barcelona to the title, though they won't give it up easily."
Ronaldo put them level in the 25th after streaking off the left flank to score from the edge of the area, but the hosts' were found wanting at the back again from an Osasuna throw in.
A nail-biting second half had Casillas saving point-blank from Aranda and Osasuna's Masoud missing an open goal before Ronaldo rose above the mediocrity to keep the title race alive with his late strike.
Gonzalo Castro, Nunes and Aritz Aduriz scored the goals that moved the islanders on to 59 points two ahead of Sevilla, who had earlier won a dress rehearsal for the King's Cup final by beating Atletico Madrid 3-1 at home.
Brazil's Luis Fabiano put Sevilla ahead from a corner in the fifth minute and almost immediately Portugal's Tiago volleyed the visitors level after a free kick.
