Sam Davies came off the bench to score 15 points and the Ospreys put in an almighty defensive shift to stun Clermont with a 21-13 win on Friday.
Dan Biggar and Morgan Parra kicked two penalties apiece in the first half, but it was the French outfit that went into the break 13-6 in front following Wesley Fofana's touchdown.
Clermont failed to build on that in the second period as Davies took to the field for the injured Biggar to score four penalties and a drop goal.
That completed an outstanding win and lifted Steve Tandy's men to the top of Pool 2.
Clermont have failed to find their usual fluency in this season's top-tier European competition with them going down to a heavy defeat at Exeter Chiefs earlier.
They also struggled to victory against tonight's opponents in the reverse fixture and the visitors found the Ospreys similarly tricky at the Liberty Stadium.
Although Fofana's converted try, following Camille Lopez's break and offload, moved the visitors ahead, the hosts were putting them under pressure in defence.
Justin Tipuric was superb and one of his surging runs set up the position for Biggar to kick his first penalty of the evening.
The fly-half then added his second off the tee before the Welsh region infringed needlessly at the restart and Parra regained the away side's four-point buffer.
A yellow card for the French outfit's openside flanker Viktor Kolelishvili then gave the Ospreys the perfect opportunity to attack.
However, Clermont held out and had actually extended the lead by the interval via the boot of their scrum-half.
Both teams struggled to create openings in the second half and the hosts appeared to suffer a blow with the departure of Biggar to injury, but talented fly-half Davies came on in his place.
He immediately made an impact by scoring a penalty and drop-goal to reduce the arrears to one point.
Clermont were now under severe pressure and, after Alun-Wyn Jones had burst through, John Ulugia was caught offside. The replacement was therefore sent to the sin bin and Davies kicked the resultant opportunity to move the Welsh team into the lead for the first time in the match.
As conditions worsened, Clermont began to overplay and the Ospreys took advantage with Davies adding successive three-pointers late on to seal the win.
Pens: Biggar 2, S Davies 4
Drop Goals: S Davies
Pens: Parra 2
Yellow Cards: Kolelishvili, Ulugia
Ospreys: 15 Dan Evans, 14 Hanno Dirksen, 13 Jonathan Spratt, 12 Josh Matavesi, 11 Eli Walker, 10 Dan Biggar, 9 Brendon Leonard, 8 James King, 7 Justin Tipuric, 6 Sam Underhill, 5 Alun Wyn Jones (c), 4 Lloyd Ashley, 3 Aaron Jarvis, 2 Scott Baldwin, 1 Paul James
Replacements: 16 Sam Parry, 17 Nicky Smith, 18 Ma’afu Fia, 19 Rory Thornton, 20 Dan Baker, 21 Tom Habberfield, 22 Sam Davies, 23 Owen Watkin
Clermont: 15 Scott Spedding, 14 David Strettle, 13 Jonathan Davies, 12 Wesley Fofana, 11 Noa Nakaitaci, 10 Camille Lopez, 9 Morgan Parra, 8 Fritz Lee, 7 Vito Kolelishvili, 6 Damien Chouly, 5 Sébastien Vahaamahina, 4 Loïc Jacquet, 3 Daniel Kotze, 2 Benjamin Kayser, 1 Raphael Chaume.
Replacements: 16 John Ulugia, 17 Vincent Debaty, 18 Michael Simutoga, 19 Paul Jedrasiak, 20 Camille Gerondeau, 21 Ludovic Radoslavjevic, 22 Brock James, 23 Albert Vulivuli
Referee: Wayne Barnes (England)
Assistant Referees: Luke Pearce (England), Peter Allan (England)
TMO: Sean Davey (England)