Jason Caron, who was tied with Purdy after the first round, carded a 3-under 69 to finish at 10-under-par 134 along with Jeff Freeman and Jason Gore.
Purdy has seen a full turnaround this week at Shadow Hills Country Club. The 29-year-old, who missed the cut in each of his last five starts, followed up a brilliant opening-round performance to hold the 36-hole lead for the first time in his Buy.Com Tour career.
'They're tucking the pins, but the greens are soft,' said Purdy. 'I was trying to shoot as low as I can. These guys are good. It's pedal to the metal and there's no reason to be tentative.'
Purdy played the back nine first and picked up his first birdie of the day at the 11th. He ran into trouble with a bogey at the 14th but responded with a birdie at the very next hole. Purdy added a birdie at the 18th to make the turn at minus 9.
The former Arizona Wildcat had the shot of the day at the par-4 sixth. He was left with 150 yards to the hole and sent his second shot into the bottom of the cup for an eagle.
'It went in like a putt. It landed 10 feet and rolled right in the hole,' said Purdy. 'Some pins you don't shoot for, but that one I was shooting for. I was in the middle of the fairway with a comfortable club.'
Purdy followed his eagle with a birdie at the par-4 seventh to take sole possession of first into the weekend.
'Golf's a crazy game. It's not surprising me that I'm playing well,' said Purdy, whose best finish this year is a tie for 23rd at the Charity Pro-Am at the Cliffs. 'What was more surprising was how bad I was playing.'
Patrick Moore, who is looking for his third victory of the season, had six birdies and no bogeys for a round of 66. Moore finished three shots back at 9-under-par 135, alongside John Restino, Ryuji Imada, Billy Judah and Brent Winston.
Jason Dufner, David Branshaw and Brad Adamonis share 11th at 8-under-par 136.
The 36-hole cut fell at 3-under-par 141 with 74 players qualifying for the weekend. Among those who didn't make the grade was Keoke Cotner, who won this event in 2000.
Full field scores from the Oregon Classic