The first real momentum swing occurred on Sean Mannion's pick-6 to Julian Gener. Before the interception Oregon State was in control of the game, they were hardly dominating, but there was little question about who would win the game. The Beavers had just forced a three and out and had only allowed 49 yards and one 1st down. After the punt the Beavs started from their own 28. Mannion faced some pass rush on a screen pass and tried to force it into a tight space to Tyler Anderson, but Gener was there. He took it back for a touchdown with no one able to catch up to him since the pass was picked off behind the line of scrimmage. All of a sudden Hawaii had life, they forced a three and out and then on their next drive they tore up the Beaver defense and the game was tied. If Hawaii had kept up this would have been more important to the result, but the last major momentum change was the Oregon State drive to start the second half.
After the Warriors late charge at the end of the first half, it seemed like Oregon State had lost control of the game. They regained that control with a four and a half minute drive covering 69 yards in 9 plays. They started running and moved to passing once that well went dry, culminating in a touchdown pass to Brandin Cooks. After this drive it seemed apparent that OSU was again in the driver's seat, especially after the defense held for a three and out right after that.
This would be the final momentum swing of the game and as such it was the most important one of the game as well, resulting in a Beaver victory. I would say that the pick-six was the largest momentum change of the game where Hawaii suddenly had a little bit of life after getting thoroughly handled for the whole first quarter.
Some other candidates for the greatest momentum swing of the game were any of the 4th down conversions by the Beavs, but other than that the game was fairly straight forward from a momentum point of view.