Forest boss Stuart Pearce, by his own admission, arrived at the iPro Stadium in a vulnerable position, having overseen just two wins in 20 matches. But victory against the Reds’ fiercest rivals, on their own patch, to deny them top spot in the Sky Bet Championship, may have bought him some breathing space.
His position looked precarious at half-time as Derby led through a Henri Lansbury own goal. But Forest emerged for the second half a different team and deservedly equalised through leading scorer Britt Assombalonga, before Osborn won it.
The first half was a tale of two corners, with luck favouring Derby on each occasion.
Forest should have had an early penalty when Jake Buxton wrestled with Michail Antonio as the ball come into the box. Referee Andrew Madley could not have seen the incident or he would surely have pointed to the spot.
Then, at the other end, after 16 minutes, Johnny Russell’s corner was headed into his own net by Lansbury at the near post.
After a fairly steady, if somewhat nervy start, Forest suddenly looked vulnerable and a succession of Derby corners followed. The anxiety of those in Red shirts was visible as each cross came into their box.
Dorus de Vries came to punch the fourth corner but the ball was returned by Jamie Ward who, after controlling on his chest and cutting by a defender onto his left foot, arrowed a shot goalwards that the Forest goalkeeper had to help over the crossbar.
Will Hughes went close with a dipping left-footed shot from the right-hand side of the area as half-time approached.
Forest had offered little – a tame Antonio effort straight at Lee Grant and a skewed half-volley by centre-half Jack Hobbs – before finally looking threatening in first-half stoppage time.
Antonio got in behind Derby left-back Craig Forsyth but covering defender Buxton slid in to beat Assombalonga to the cross inside the six-yard box.
Assombalonga then hit a volley that had Pearce bouncing up and down on his tip-toes in anticipation, and hope, in his technical area, but the ball looped just over the crossbar.
Forest had finished the half with a modicum of success and they built on that after the break.
The Reds were beginning to show the fighting spirit Pearce had called for following last weekend’s home defeat to Sheffield Wednesday, and in the build-up to this game.
Buxton threw himself in front of a blast by Antonio, saving a certain goal three yards in front of the line following a scramble.
Forest continued to put Derby under pressure and, after more ricochets inside the hosts’ penalty area, centre-back Kelvin Wilson scuffed a shot that Grant was forced to save.
The Rams had lost their way and were struggling to cope with balls into their box, so it was no surprise that was the source of the equalising goal.
Steve McClaren’s men had their warnings with the previous two scrambles and, on the third occasion, they were punished.
Once more no-one in the Derby defence took command of the situation and the ball trickled through a sea of legs to Assombalonga, who spun and shot past Grant from inside the six-yard box.
You could see what it meant to Pearce as he ran, arms aloft, to the edge of his technical area in celebration.
But a further twist in the tale remained. Forest were not done yet. They felt they could win the match – and they were right. Osborn led an injury-time break down the left before thundering a shot past Grant from the edge of the box.
This time Pearce celebrated on the pitch.