Spooner's winner came 3:20 into the extra session when he took a feed from Torey Krug and slipped one through Carey Price's five-hole to secure the victory.
"Felt great. The offense in the last 15 games…haven't been able to get a lot so happy for that, was happy that we won, too," said Spooner. "The two guys actually tried to pinch me off at the blue line. I kind of panicked a little bit and threw the puck in the air.
"Tried to beat my guy to the net and it was a great pass. I came in and I was going to go backhand, saw that he was playing deep and just tried to fake a shot."
As the Bruins celebrated, the play was reviewed to check for offsides. It was the second review of the extra session. Earlier in overtime David Krejci appeared to score, but the officials on the ice called goaltender interference on David Pastrnak, who tumbled into Price.
"It was great to be back on track," said Patrice Bergeron. "We talked about that, we talked about going back to playing our game and playing our style. I thought it was a great effort.
"Obviously we gave them a point with three minutes left, but we showed some character staying with it and having some chances in overtime and finally putting it in."
Czarnik, who was back at his natural center position, ended over a month of frustration with 1:05 left in the second period. The rookie was robbed of the tying goal against Toronto on Saturday night and had another golden chance early against Montreal.
He finally broke through when he sped down the left wing and fired a shot short side on Price to give the Bruins a 1-0 lead. It was Czarnik's first goal since Nov. 10. Adam McQuaid registered the assist for his first point of the season.