Spanish giants fall seven points behind leaders Barcelona...
Camunas' second-half strike was enough for the hosts to pick up three valuable points in their fight against relegation and also saw Madrid fall seven points behind league leaders Barcelona.
The first chance of the evening fell to Osasuna's Coro as early as the third minute but his effort fell wide of Iker Casillas' goal. Minutes later Nelson sent in a teasing cross that Walter Pandiani failed to get a proper touch on from close range.
Seconds later, though, it was Karim Benzema's turn to fail to find contact with the ball as a beautifully executed right-wing cross went to waste.
Madrid were controlling the possession but Osasuna were having the chances fall their way. The Pamplona-based side could have broken the deadlock just after the 15 minute mark, had Sergio Fernandez's header from the centre of the Madrid penalty box not been saved.
An Osasuna cross from the left ignited miscommunication inside the Blancos area but the ball eventually bounced off Casillas' arm with no opposition player near him to take advantage of the hesitancy of the Spanish goalkeeper.
Jose Mourinho's side crafted a couple of chances around the half-hour mark. First Angel Di Maria's left-footed shot from outside the hosts' box went wide and then Benzema tried an audacious volley from a brilliant left-wing long ball from Mesut Oezil but couldn't hit the target.
It could have proved costly for Madrid had Pandiani not been in a benevolent mood as the Uruguayan forward failed to keep his free header from a Camunas cross on target from just yards out.
Madrid were sparked into urgency thereafter as they finished the half the stronger of the two sides, with Benzema and Ronaldo both forcing Osasuna goalkeeper Ricardo into making brilliant saves.
Di Maria too tried his luck and so again did the Portuguese attacker, who had earlier been robbed of a great chance when he was dispossessed by Sergio inside the Osasuna area.
The second-half commenced with Nelson going for a long-range effort but in vain. Madrid then took over the proceedings as Ricardo Carvalho went close with a header and Di Maria's shot went wide.
The hosts broke the deadlock just after the hour-mark virtually without any warning through Camunas. Madrid's defence failed to stay alert and Carlos Aranda threaded the ball between a host of legs to Camunas, who finished brilliantly from close range.
Mourinho made a triple change, bringing on Kaka, Xabi Alonso and Manchester City loanee Emmanuel Adebyor, who made his first appearance for Madrid.
But it was Oezil who could have restored parity with a free-kick on 70 minutes. However, the German international midfielder was unlucky not to have found the net with his well-taken strike.
Oezil then played a brilliant ball to Benzema but the French forward's header went over the Osasuna crossbar. Minutes later Adebayor saw his header saved.
Ronaldo and Kaka both decided to take long-range strikes with the former drawing a save from Ricardo and the latter hitting his shot wide, before Alvaro Arbeloa cleared the ball off the line to deny Krisztian Vadócz his goal.
Madrid cotinued to chase the equaliser but eventually failed, meaning that they are now seven points off the pace and have virtually kissed their title dreams goodbye.