Tim O'Brien scored with 1:38 left in overtime Friday as Dartmouth rallied from a 3-0 deficit to stun eighth-ranked Union, 4-3, Friday at Messa Rink.
The Dutchmen blew a 3-0 lead.
Brad Schierhorn scored with 49.9 seconds left in the third period to tie it.
Union had the 3-0 lead thanks to a first-period goal by Tyler Hynes and second-period tallies by Max Novak and Spencer Foo. Novak's goal came short-handed.
Eric Robinson started the Big Green's comeback when he scored a power-play goal late in the second. Tyler Skiura made it 3-2 4:40 into the third.