At the 18th hole at El Cid Resort, both players had close to 15 feet for birdie. Oppenheim buried his birdie try to win and it was his second title of the year.
He overcame a four-stroke deficit to win the Barton Creek Classic earlier this season, then on Sunday, Oppenheim erased a seven-shot deficit.
'It's just a crazy game, isn't it?' said Oppenheim, who is first on the Canadian Tour money list. 'I really wasn't thinking about winning the tournament. I just wanted to go out and post a number.'
The number he posted on Sunday was an 8-under 64 to join Gangluff in the lead at 15-under-par 273. Gangluff only managed a 1-under 71 in Sunday's final round.
Chris Wall carded a 3-under 69 on Sunday and finished alone in third place at minus-14. Michael Walton (70) and Josh Habig (69) tied for fourth place at 12-under-par 276.
Gangluff did not play spectacularly on Sunday, but he held a two-shot lead with three holes to go. He bogeyed the 16th, then his third shot at the par-5 17th plugged in a bunker and he could not save par.
Oppenheim was informed he was heading to a playoff and eventually won, but he felt for Gangluff. At Gangluff's last start, he went into the final round with the lead, but finished fourth.
'I really feel bad for Steve,' admitted Oppenheim. 'He is such a great guy and he was so close again. But that's golf. Next time, I might be where he is.'
'That Oppenheim loves those birdies on the first hole, doesn't he?', said Gangluff, referring to how Oppenheim won in Texas. 'This one was a little easier to take. I played well, but you can't shoot one-under on the final day and expect to win the tournament. I had it in me, but came up short. That's the way it goes.'
Oppenheim bogeyed the sixth, but closed his front nine in style with three straight birdies from the seventh. He birdied three of his first four on the back side, then traded a bogey and a birdie over his last four holes.
Anthony Rodriguez (67), Brian Miller (71) and Wil Collins (74) shared sixth place at 11-under-par 277.