Martin Jones made 33 saves for the win, while Eddie Pasquale stopped 22 shots for the IceCaps.
O'Dell set up Cormier's power-play goal at 16:26 to put St. John's ahead 1-0 in the first. Cormier, who made space for himself in front of the net, took the feed from O'Dell, who held the puck and drifted by Jones before passing.
The IceCaps got out to a quick start in the second period when O'Dell scored just 12 seconds in.
O'Dell gained the zone and let a slap shot go from the slot.
Manchester replied almost immediately when they scored at 1:36 when Kozun put home a one-timer through traffic to bring the Monarchs within one.
St. John's regained their two-goal cushion on O'Dell's second goal of the night at 3:34 of the second.
Jones misplayed the puck, and made multiple saves, before a crafty pass from behind the net found O'Dell all alone in front of the net.
Linden Vey set up Toffoli's breakaway goal at 13:17 of the third. Toffoli faked a shot and used a forehand-backhand deke to get the puck past Pasquale.
The IceCaps re-established their two-goal lead at 14:21.
Mouillierat was waiting on the back post when a puck trickled through the crease and he jammed it in.
A bad give-away in the IceCaps' zone from Aaron Gagnon led to a power-play goal at 16:15 from Andreoff.
Andreoff's second of the night at 19:17 of the third sent the game to overtime.
St. John's was 3 for 6 on the power play and Manchester was 2 for 5.