Clermont captain Damien Chouly has admitted that his side's fate in the Champions Cup depends on other results after losing on Friday.
Les Jaunards looked in control by half-time with their set-piece on top and leading 13-6 at the break.
But a spirited second half performance from the Ospreys both meant that Clermont lost the match at the Liberty Stadium and failed to secure a losing bonus point.
Last year's finalists now drop down into second place and will have their eyes on Saturday's game between Bordeaux-Bègles and Exeter Chiefs to see what happens next in Pool 2.
"They scored every time they got into our half, we could not keep the ball under control in the second half and gave them opportunities to score," Chouly told the EPC Rugby website.
"Every time they attacked they seemed to get a penalty or they kicked a drop goal and they put the pressure on us.
"We have lost control of our destiny, we have one game left which we have to win and we will have to wait for the result in the other games. We were naïve for half of that game, we gave them ball.
"We made too many mistakes and it proved very costly for us."