You may be worrying too much about Osasuna. Anything can happen on the field, that's the future. But in terms of position and performance, Osasuna is not a big threat to Barcelona. Just because Osasuna has a small chance of winning doesn't mean that Osasuna is going to win the match. I can say for sure that if you were asked to bet on Osasuna, you would definitely refuse, because you yourself know how likely Barcelona is to win. If Barca loses to Osasuna, I would definitely consider it an accident and such an accident is part of the game or a normal occurrence.
Towards the end of the season, there usually are some surprising outcomes. You would see a team who is struggling to avoid relegations beating highly placed clubs. Looking at the La Liga standing it is obvious that Osasuna has not yet escaped relegation; that is why every single game will be important to them. Barcelona are favourites to win this game and I will definitely bet on them but that does not erase the fact that the match could end in a draw or that Osasuna can pull a surprise.