Ryan Stoa also had a goal and Kyle Quincey had two assists for the Avalanche, who won for the first time in three preseason games.
Richard Bachman had 28 saves for Dallas, which fell to 3-1 in the preseason.
The Avalanche scored their first goal on their first power-play of the game. Joakim Lindstrom took the puck at the point, maneuvered to find an open shooting lane and sent a shot to the front of the net. The puck bounced off Stoa at the top of the crease and trickled through the legs of Bachman, who was screened on the play.
Early in the second period, Gabriel Landeskog, the second overall pick in the June draft, carried the puck down the right side into the Dallas zone early and feathered a pass to O'Reilly, who had gotten behind the Stars' defense. O'Reilly lifted a shot over Bachman's right shoulder to give Colorado a 2-0 lead 2:24 into the frame.
O'Reilly banged home Quincy's rebound shot to make it 3-0 late in the period.
Giguere faced an early test when Dallas had a 5-on-3 power play for 1:22 just 73 seconds into the game. But Dallas managed just one shot on the two-man advantage and Giguere easily stopped it.
Later in the period Dallas forward Krys Barch took a pass from Jake Dowell and came in alone on Giguere, but the goaltender stuffed the shot attempt to keep Dallas off the scoreboard.
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DENVER, CO - SEPTEMBER 25: Gabriel Landeskog #92 of the Colorado Avalanche warms up prior to facing the Dallas...