Elsewhere, Duncan Stewart is two shots off the lead in Kazakhstan, while Chris Doak and Scott Jamieson are still in with a shout in the final round of the Italian Open.
It is Oldcorn, though, who is in the most commanding position. The Scot's four-under-par 68 took him to a nine-under-par total of 135 after errors by Luis Carbonetti and Philip Golding dented their hopes at Terre Blanche Hotel Spa Golf Resort.
Argentine Carbonetti had shared the overnight lead with Oldcorn, and the pair spent most of the day on equal terms, but a double-bogey five on the 16th hole, followed by bogeys on the 17th and 18th, meant Carbonetti signed for a 73 to slip five shots off the pace.
Golding was also in touch until he lost his ball off the tee on the last hole and racked up a double-bogey six to finish, the Englishman posting a level-par 72 to also end up on four under par.
Oldcorn, meanwhile, ironed out the mistakes he made on his front nine, when he mixed bogeys on the fourth and seventh holes with birdies on the second and sixth, as he came back in 32 shots. Having finished in the top 10 in his last three Senior Tour events, he birdied the 11th, 12th, 14th and 17th holes in a flawless back nine to open up a cushion at the top of the leaderboard.
"It was a funny day. I didn't strike the ball as well as yesterday, and didn't putt as well - certainly the front nine. But I stuck at it. I knew I was still swinging pretty well and that opportunities would come on the back nine," Oldcorn said.
Australian Peter Fowler (71) and South Africa's Chris Williams (69) are Oldcorn's nearest rivals on five-under par, but even the Scot admitted he was surprised by the margin of his lead following a tight battle with Carbonetti for 15 holes.
"There was a massive swing for me," said Oldcorn. "Luis had dragged me along before that because he played really well. I was trying to stick with him. So to come out with a four-stroke lead is great. It would have been inconceivable standing on the 16th tee, but that is golf. I'm taking nothing for granted. I like where I am and know what I have to do."
In the Kazakhstan Open, meanwhile, Stewart birdied his final three holes to card a seven-under-par 65 and move within two of the leader going into the final day at Nurtau Golf Club.
The Grantown-on-Spey man, in his first season on the European Challenge Tour, had begun the day five shots off the pace but birdies at the par-5 second and par-3 fifth - secured with an 18ft putt - took him to the turn in two under par. He then birdied the 10th, 11th and 16th (with a 35-footer), and continued all the way to the close.
"All day the putter was just red-hot," said the 29-year-old. "I played really solidly as well, got really good up-and-downs at the seventh and 15th which are probably the two hardest holes."
Stewart is two shots behind Johan Carlsson in the €400,000 Almaty event after the Swede signed for a 67.
At the Italian Open, Australia's Marcus Fraser leads by one shot from a group including local favourite Francesco Molinari.
Chris Doak shot a 69 to leave him on six under par, five shots off the pace, with Scott Jamieson a shot further behind after a round of 70.