Hershey (18-3-3-4) had its four-game winning streak halted,
Toronto had an apparent tying goal disallowed two minutes into the third period when the net was dislodged before the puck crossed the goal line. Westrum slammed a rebound home on a power play midway through the period to make it 3-3.
The Bears outshot the Marlies 4-2 in the overtime.
After a scoreless first period, Toronto needed just 56 seconds in the second to take a 1-0 lead. John Mitchell took a pass from Brett Englehardt and beat Frederic Cassivi.
Chris Bourque tied it for Hershey, converting a rebound of his own shot 6:44. Dave Steckel made it 2-1, converting a rebound of a shot by Bourque.
Toronto tied it 25 seconds later on the third goal of the year by Jamie Sifers. Bourque regained the lead for the Bears with 2:18 remaining in the period on a perfect pass from Matt Hendricks at the top of the crease.