I tend to advice not to take any results for granted, and not to underestimate any rivals neither. But, in this case, I would dare to make an exception and bet for Real Madrid, as it is a clear win to me. And don't take me wrong, I like Osasuna, and they are performing really well this season (fifth place so far is a really good result for them), but because of that, Real Madrid will be well prepared and it will be difficult to catch them off guard.
It's not about underestimate their opponent
But whatever the reason, based on the performance of Real Madrid it is clearly far superior to Osasuna, as well as the mentality of the Madrid players because based on the history of meeting the two clubs, Osasuna has never even win matches against Real Madrid since the last few seasons.
The only good result for Osasuna was a draw, but I know it will be difficult next week because Real Madrid are in an insecure position as Barcelona continue to follow behind and at any moment can pass Madrid in the table if they make a mistake, this will force Madrid to play optimally to defend their position.