Bryant had 11 birdies and one bogey for a three-shot lead Thursday in the fourth of five majors this year on the Champions Tour.
Bryants best finish this year was fourth at the Dicks Sporting Goods Open in June. He has four overall tour victories, his last coming at the Senior U.S. Open in 2007.
Loren Roberts shot a 65, while Tom Watson and Tom Lehman each had 67s as temperatures hovered in the upper 90s on the scenic 7,533-yard course in the shadow of Mt. Bachelor.
Defending champion Fred Funk was in a group at 4 under. Tour money leader Bernhard Langer shot a 71.
Even Bryant was stunned by his low round, which matched Watsons low second round in 2003 when the tournament was played in Aloha, Ore., west of Portland.
Bryants personal best round was 63.
Just played really great, he said. Definitely one of my top five ball-striking rounds in my life.
Bryant won once on the PGA Tour, at the Walt Disney World-Oldsmobile Classic in 1995.
Ten under? Roberts mused. I just cant believe how good that is.
Roberts, who has won twice on the tour this year, had five birdies and an eagle on the par-4, 410-yard 10th hole. Roberts won the Tradition 2005.
Watson, who won the Tradition in 2003, fell to Stewart Cink in a dramatic playoff at the British Open a month ago.
It was a pretty benign day, Watson said. Not a lot of wind to deal with. The flags were accessible.
However, the altitude and heat did play a role when it came to distance: That ball can get out there, it can go, he said.
Fan favorite Greg Norman withdrew shortly before the first round because of back spasms. It would have been the Sharks first appearance in the event, and sixth start on the tour this year.
Funk was coming off a victory at the U.S. Senior Open, where he became the first player in a USGA championship to finish at 20-under par. A week earlier, he lost in a three-way playoff at the Senior British Open.
He has finished in the top 10 in the previous three Champions Tour majors this season and leads the Charles Schwab Cup points standings.
Thats my goal this year, to win the Charles Schwab Cup. I really want it, Funk said.
Funk shot a final-round 3-under 69 in the Tradition last year for a three-shot victory over Mike Goodes and his first win in a major on the tour.
This year, his first round featured four birdies.