Gloucester coach Dean Ryan said his side lost to the Ospreys because of "an appalling first 20 minutes".
Ryan was left to fume on the sideline as his side - the only unbeaten team in the Heineken Cup going into the fifth round - were comprehensively beaten 32-15 at a rain-soaked Liberty Stadium.
"The Ospreys took everything that was on offer, and there was a lot because our first 20 minutes were appalling.
"We were left to chase the game and you can't do that in those conditions. "
Man of the match, Ospreys scrum-half Justin Marshall, said: "This is a hugely important result.
"We were up against a crack side, but we knew we could produce a win for our supporters. It was emphatic, but it was a lot of hard work. The forwards were a pleasure to play behind in such awful conditions.
"James Hook kept Gloucester out of touch - his goal-kicking was outstanding."