Larkin ended the 3-on-3 overtime, carrying the puck inside the right circle and scoring on a wrist shot that got past Veini Vehvilainen's blocker. The Finns complained about an open-ice hit that wasn't ruled a penalty against the U.S. shortly before Larkin scored the game-winning goal.
Brady Skjei scored in the opening minute and Johnny Gaudreau put the Americans up 2-0 midway through the third period. The Finns rallied to tie the game 2-all with Harri Pesonen's goal in the last minute of the opening period and Niko Ojamaki's goal in the final minute of the second.
The matchup in Group A featured American center Jack Hughes and Finnish winger Kaapo Kakko, who are expected to be picked No. 1 and 2 overall, respectively, in the NHL draft next month.
Mark Stone scored with 1.8 seconds left to lift Canada over Slovakia 6-5, prompting furious supporters of the host country to throw coins and other objects on the ice.
The Canadians rallied to win after trailing 2-0 in the first period and 4-2 in the second.
Matus Sukel's second goal pulled the Slovaks into a 5-all tie midway through the third period, but his teammate, Erik Cernak, was called for tripping with 1:14 left and Stone took advantage. He scored on a wrist shot from the right circle, triggering a celebration on his 27th birthday.
Slovakian fans responded with a chorus of boos, and some emptied their pockets and chucked their spare change onto the playing surface.
Canada improved to 2-1 and dropped Slovakia to 1-2 in Group A.
Russia stayed undefeated in Group B and handed the Czech Republic its first loss, winning 3-0 in Bratislava.
Sergei Andronov scored midway through the first period and Nikita Gusev gave the Russians a 2-0 lead in the middle of the second. Nikita Zaitsev added an empty-net goal late in the game and Andrei Vasilevskiy finished with a 23-save shutout.
The Russians are 3-0, outscoring opponents 13-2.
Patric Hornqvist scored twice to help Sweden rout winless Norway 9-1. The two-time defending champions improved to 2-1 in Group B.
The Associated Press contributed to this report.