Both teams headed into the game in the bottom three, and it was the visitors who clinched the points which saw them leap out of the drop zone and to the relative security of 14th place.
Oriol Riera set the Pamplona side on their way in the 19th minute and Alejandro Arribas gave them breathing space 13 minutes later.
Fernando Soriano pulled a goal back against his former club 17 minutes from time, but there was no way back for the hosts - whose evening got even worse at the death with the dismissal of Rafita for two stoppage-time bookable offences.
Barring a couple of exceptions - most notably in 2006 when they finished fourth - Osasuna have spent much of their time since winning promotion to the top flight in 2000 simply looking to survive.
They have managed to do that for 13 successive seasons now, although they have had more than a few close shaves, and could be on the verge of pulling off another great escape after Friday's morale-boosting away win.
Riera opened the scoring from their first attack of note, lashing high into the centre of the goal from Alvaro Cejudo's pass.
Arribas doubled the visitors' advantage just after the half-hour mark, when he was afforded too much time and space and headed a Roberto Torres free-kick into the bottom-right corner.
The best Almeria could muster during the first half was a couple of speculative long-range efforts from Jose Antonio Verza which sailed harmlessly over the bar.
The home team were almost handed a way back into the game 11 minutes after the restart when Osasuna goalkeeper Andres Fernandez fumbled a Suso blast, but he got away with it.
Fernandez did rather better soon after to parry a Ramon Azeez blast - only for the ball to fall kindly for Soriano, who despatched the rebound to halve the deficit in the 73rd minute.
Osasuna lost goalscorer Riera through injury in the closing stages, but never looked in too much danger of surrendering their lead - particularly with Almeria reduced to 10 men before the final whistle.
Rafita was first booked for a challenge on Riera's replacement, Javier Acuna. He then followed that up with a foul on another substitute, Oier Sanjurjo, and received his marching orders.