The Ducks started the game shooting a blistering 12 of 16 from the floor and led 31-12 after a Jason Calliste 3-pointer at the 10:39 mark. Oregon went 7 for 7 on 3-pointers during the stretch, with Moser making three.
"For a while there, we were 100 percent. You just didn't want to be that guy to miss," said Moser, who finished with 15 points and seven rebounds.
"Taking good shots, good shooters were shooting the ball, good passes. Just knocking it down," said Ducks point guard Johnathan Loyd.
Joseph Young scored 25 points for Oregon (16-8, 4-8 Pac-12), which had five players in double figures.
Eric Moreland led Oregon State with 17 points and 10 rebounds, while Roberto Nelson scored 18 for the Beavers (13-11, 5-7 Pac-12). Oregon State now has lost three in a row.
"You can't play any game, let alone the Civil War game, where you start the game off and give them a 19-point lead," said Oregon State coach Craig Robinson.
After falling behind early, Oregon State responded with a 15-3 run, making it 34-27 after a Nelson breakaway lay-in with 5:13 left in the half.
But the Ducks pulled away again and took a 45-35 advantage into the break.
In the second half, the Beavers twice trimmed the lead to two points, the latter when Hallice Cooke sank two free throws to make the score 72-70 with 5:11 remaining.
"That's a lot of character there," Robinson said. "That's not just basketball. You have to know how not to give up and keep chipping away and sticking with your game plan to make that kind of comeback."
Missed shots and offensive fouls on two straight possessions doomed any hope of a comeback for the Beavers, and the Ducks went on a 9-0 run capped by another Calliste 3-pointer with 2:51 left.
"We just wanted to find a way to stop them and rebound down the stretch," Moser said.
"When they got it back to a two-point game, our guys struggled a little bit with that, but we had some guys make some plays," said Oregon coach Dana Altman.
Oregon made 12 of 14 free throws in the final 2 minutes to seal the game.
The Ducks were 11 of 21 on 3-pointers. Ironically, long-range shooting has been a strength for the Beavers this season.
Oregon State entered the game ranked first in the Pac-12 and fifth in the nation for 3-point shooting at 41.4 percent. That figure would be a school record if it holds for the rest of the year.
The Beavers hit 5 of 16 from beyond the arc on Sunday.
No college basketball teams have played more against each other than the Ducks and Beavers, whose game Sunday was the 341st matchup in the "Civil War" series. Oregon State leads the series 185-156.
Young, Oregon's leading scorer, had just five points in the Ducks 80-72 loss to the Beavers on Jan. 19. OregonState held Oregon to 38 percent shooting in that game.
On Sunday, the Ducks shot 53 percent from the floor, compared to nearly 52 percent for the Beavers.
The Ducks started the season 13-0 before losing five of their first six games in conference play. The Ducks had dropped three of their previous four games, and all of the losses came by two points.
Altman said Oregon isn't a tournament team right now. "But we can change that. We're going to have plenty of opportunities, if you look at our schedule," he said.
Four of the Ducks six games before the Pac-12 tournament are at home.
With the win, Loyd is tied for the most victories ever in a Duck uniform with 89. Former teammate E.J. Singler shares the record.
Loyd had 11 points and five assists on Sunday. "This is my last Civil War, so I definitely wanted this game," Loyd said.
Nelson entered the game leading the Beavers and the Pac-12 with 21.5 points per game.
The Beavers were looking for just their second season sweep over the Ducks since 1993. The last was in 2010.
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