"Was a tough penalty by me, I can't be taking one like that to give them a five-on-three," O'Reilly said after the game, per KSDK News. "Obviously they got that goal and took that momentum from us."
With the Blues already shorthanded in the first period, O'Reilly was whistled for a delay of game penalty after accidentally flipping the puck over the glass. Bruins forward Brad Marchand capitalized on the two-man advantage, ripping a one-timer past Jordan Binnington to give Boston a lead they never relinquished.
"They got a lucky bounce on that second goal and it took the wind out of our sails," O'Reilly said.
The Blues had four power-play opportunities of their own but failed to execute despite mustering 12 shots on Bruins netminder Tuukka Rask. St. Louis has now gone 1-for-18 with the man advantage in the series.
"Some good looks, just not the results," O'Reilly said. "Maybe if we do get one it steamrolls into two or three and it's unfortunate, but we've got to build and it's going to take a lot of hard work to bury and we're not afraid of it."
O'Reilly scored the Blues' lone tally in the loss and has a team-leading four goals and seven points in the Stanley Cup Final.
For the first time since 2011, the Stanley Cup will be decided in a winner-take-all contest as the two teams head back to Boston for Game 7 on Wednesday.