Osasuna is a good team, but we are talking about weakness in relation to Real Madrid, right? The fact that they are in fifth place does not mean that they are "close" to Real Madrid in terms of strength. No matter how badly Real Madrid's opponents play, there will always be someone in second place. Even if Real Madrid is a million times stronger than everyone else, someone will be in second place, third, and so on. I am sure that if this game was in the Champions League final and it was the last game of the championship (there would be no need to save energy), then Real would have won 10 games out of 10 and the goal difference would have been consistently 3+.
IMO, Osasuna is not bad, but not a good team neither. The fact that they are in the fifth place right now is more accidental than due to technical of physical competitiveness. We are still at the start of La Liga, so luck plays an important role in the ranking. Let's see if they manage to stay in the top 10 when we approach the end of the season. Their great start will help, but it is not a sure thing.
Yes, given that Sevilla has collapsed and Atlético are still looking for their game, any place below second is vacant for any team. Especially at the start of the season, when the factor of easy calendar and "luck" plays a big role. The further the championship goes, the more familiar the standings will take (although I'm not sure that Sevilla will be able to return to the top half).