Moments before scoring his second goal, Osimhen was denied by a superb save from Forster, but there was nothing the Tottenham goalkeeper could've done there.
The ball into the penalty area was delicious from Mertens and all it needed was a deft left-footed touch from Osimhen. A brilliant goal from the home side but the defending, particularly from Dragusin, has been awful from the visitors.
The momentum swings in this game are absolutely fascinating.
That equaliser had given Tottenham renewed hope but Galatasaray have stolen it back emphatically. All of a sudden, Spurs are on the ropes with their hosts suffocating them every time they get the ball.
There was a brief VAR check for a red card on Bissouma a moment or two ago, but it didn't come to anything.
Dragusin will want the ground to swallow him up.
He was caught in possession 30 yards from goal and Osimhen smelt blood. Mertens played a sharp pass through to the Napoli loanee who took one touch before stabbing the ball past Forster and into the bottom corner.
To compound Dragusin's misery, he was shown a yellow card for a foul after losing the ball.
One to forget for Tottenham's Romanian centre-back, that.
A huge roar inside Rams Park as the home supporters thought Osimhen had restored their advantage from close range. But the Nigerian's celebrations are very quickly cut short by the linesman's flag.
A swift VAR check followed and confirmed the on-field decision.
This has been a really enjoyable opening half hour.
Spurs struggled early on but have found a foothold since levelling through Lankshear. But as they started to grow into the game, Osimhen had that big chance that I just told you about.
Both teams carrying a real threat in attack with two less-than-impressive defences — I'd be shocked if this stays 1-1!
A couple of big chances for Galatasaray.
Victor Osimhen skipped into oceans of space left behind Radu Dragusin, but Forster got down well to keep out his low, curling effort. On the rebound, Mertens fired wide on his left foot from the edge of the box.
Osimhen will be disappointed with himself that he didn't find the back of the net there, but it was a decent stop from Tottenham's 36-year-old goalkeeper.
What a moment for the 19-year-old in such a hostile atmosphere!
The credit has to go to Archie Gray who floated a brilliant pass into Brennan Johnson at the far post. He kept his composure well to centre the ball for Lankshear who had the simple task of poking into the empty net from a few yards out.
Excellent football from the visitors who have responded perfectly to going behind early on.
I can barely hear myself think let alone have a conversation with the other journalists sat next to me.
It must be overwhelming to be on the pitch which means it is understandable Spurs have started slowly. They are having trouble playing out from the back and at times are just hitting it long for Will Lankshear or Brennan Johnson to chase.
They need Bissouma, Maddison and Bergvall to show more composure in midfield.
The Galatasaray tails are up and Akgun's eyes lit up again when the ball dropped to him just outside the penalty area.
This time, though, his effort was wild and never troubling Forster.
Certainly can't blame him for having another go after seeing his first shot.
Of all the places you can go a goal behind in the first few minutes, this isn't one of the better ones.
The last thing these Tottenham players need is the home fans in full voice and that opening goal has turned the volume up a notch.
This is now a real test of Postecoglou's young side as they look to strike back quickly.
I'm not sure how some of these fans in Europe do it — the home supporters are violently whistling and booing every time a Tottenham player touches the ball.
Only one way for the visitors to shut them up...