This page off their website tells you how -
http://www.fcbarcelona.com/eng/noticias/noticias/n04091410.shtml You can buy them online or from the club's ticket outlets. The game you want to see against Osasuna won't be a sellout, because although they're not a bad team, they are from Pamplona and are not well supported, and don't have any big name players.
Important: Make sure you check in advance what day and time the match is actually played on because in Spain they play league games on Saturday and Sunday evenings and often change the dates and times around.