The Australian struck a four-under par 68 in the third round to move to 12 under for the tournament and wipe out Poulter's six-shot lead, with the Englishman coming home three over to sit tied second with Lee Westwood and Marcel Siem.
After a birdie and bogey on his opening three holes, Ormsby tightened his game up nicely and by the turn he had collected another shot to move to nine under overall.
After five unblemished holes after the turn, a birdie on 15 and an eagle three at the last gave him the overnight lead.
"It feels great, to be leading makes me pretty happy," Ormsby said. "My game is good, I'm hitting strong and my putting is good. But I still feel I've left a few shots out there.
"I will just keep trying to play my own game and see how things end up."
Poulter endured a long day on the course having had to finish his second round earlier in the day.
He negotiated the final four holes easily, but his start to the penultimate round left him with much to do.
He bogeyed the first and fifth on the front nine, followed by another at the 11th, but he remained ahead until a wayward wedge at the 15th saw him card a double bogey and drop out of the lead.
A birdie at the 17th ensured he finished the day joint second, but he was left to rue a frustrating round.
"A little disappointing, [my] rhythm was off right from the get go," said the Englishman. "I wasn't aggressive enough, I made mistakes, notably on 15.
"A real lack of judgement there, it led to a double which was a real shame, as bad as it was it wouldn't have been that bad without the double bogey there."
Westwood charged into contention by shooting 67, but could have been top of the leaderboard had he not picked up two bogeys on his way round.
With Race for Dubai points still available the man to watch on Sunday is Marcel Siem, the German needs to win in Turkey to have a chance of usurping Rory McIlory, and four birdies on the back nine saw him card a 66 on Saturday.