Both teams finished with 10 men at Estadio El Sadar on Saturday, as Real's five-match winning run in the league was brought to an end.
The hosts took the lead when Oriol Riera glanced a header home from Marc Bertran's sublime centre with the outside of his foot.
Riera then doubled the advantage, heading home a rebound from close range, and when Sergio Ramos was shown his 18th career red card for an elbow on the Riera, Osasuna were in a commanding position.
But the visitors refused to buckle and halved the arrears shortly before half-time with a drilled effort from Isco before Francisco Silva's was sent off for a second bookable offence and Pepe duly delivered an equaliser to leave Ancelotti with mixed emotions.
"We lost control after the first goal," he said. "The first 20 minutes were very good. (We played) with quality and pace.
"It's amazing that we were 2-0 (down). It is difficult to explain. We've missed a lot (of chances).
"We had less balance but were very intense. The team showed a lot of character and attitude not to lose."
