O'Reilly was the fifth skater to score in the shootout as he made a move and lifted the puck over Ilya Bryzgalov. Matt Read had the last shot of the game and beat Jean-Sebastien Giguere, but rang his wrister off the crossbar.
T.J. Galiardi and Gabriel Landeskog each had a goal for the Avalanche, who have won three of their last four games. Giguere stopped 31 shots in the win.
"It's a big two points for us and we sure worked hard for it tonight," said Landeskog. "They are a very good team and they came out flying. We didn't match their work ethic at first, but we came out in the end."
The first goal of the game went to Philadelphia when Jakub Voracek got the puck down low at the right side and sent it over to the left, where Hartnell had the easy slam with 90 seconds left in the opening frame.
Colorado, though, got two goals in the second period with the first one coming at the 12:26 mark.
Shane O'Brien held the puck in the high slot and snapped a pass down low, where Landeskog shifted to the backhand and put it in.
"We were back in our zone for two or three shifts and gave them that first goal," said Philly winger Danny Briere.
In the final minute of the frame, Brad Malone fired a backhand pass from the left circle that appeared to be headed for the right point, but it hit off a Flyer, then Galiardi and went into the net.
In the final minute of the third period, Philly had Bryzgalov pulled and tied the game. A scrum at the right side saw Briere work the puck free and pull it over for the wraparound at the left. Simmonds then got his stick on it with all eyes still looking at the right and just 18.1 seconds showing on the clock.
The Flyers play in Dallas on Wednesday...Colorado hosts St. Louis on Wednesday...The Flyers took the only meeting between the two clubs last season, a 4-2 decision in Philly on Oct. 11...The Flyers' last win in Colorado was on Dec. 27, 2002...Philly was 1-for-4 on the power play while Colorado was 0-for-2.