Notre Dame fell to 2-3-1 and dropped out of the Top 10 in the Soccer America Top 25 rankings to No. 14.
Defensive breakdowns have cost the Irish dearly -- they have given up 13 goals in their last four games -- and a turnover led to Oklahoma State's first goal by Siera Strawser in the 22nd minute. The sophomore raced free into the left side of the box and launched a 16-yard shot over freshman goalkeeper Nikki Weiss into the far-right side of the net for her first goal of the season.
Kerri Hanks, the 2006 Hermann Trophy winner, tied the game in the 69th minute, scoring on a free kick for the second straight game, arching the ball around the wall and into the upper right corner of the net from 25 yards.
Notre Dame continued to play without injured center back Haley Ford while the team's other center back, junior Carrie Dew, still has yet to return to the form she displayed prior to suffering an ACL knee injury late in the 2006 season.
Oklahoma State made a noteworthy formation change in Sunday's game by shifting its top player -- Swedish All-American Yolanda Odenyo -- from the midfield to center back. Odenyo denied Amanda Cinalli's header with a goal line clearance midway through the second half.
Goalie Erin Stigler (eight saves) was also outstanding for Oklahoma State, which won the game when Kasey Langdon sent in a ball from the left side that Weiss knocked down but the Irish defense again failed to clear. Jackson rushed in to poke home her team-leading fourth goal of the season.
OSU coach Colin Carmichael called the win the biggest in the program's history.
"It is the biggest because of the tradition Notre Dame has and how hard it is to beat them here," Carmichael said. "[Notre Dame] is the type of program we aspire to be, one that is competing for championships every year. Beating them just shows the heart and desire and spirit of our kids. We stuck to our game plan, and it paid off."
The win came two days after the Cowgirls fell from the unbeaten ranks when they lost to Washington State, 1-0. WSU won the Inn at Saint Mary's Classic, with a 2-1 win over Princeto in overtime.
Inn At Saint Maryâ¬"s Class All-Tournament Team
Brynn Bemis, Washington State (Defensive MVP and Goalkeeper MVP)
Kerri Hanks, Notre Dame (Offensive MVP)
Kiersten Dallstream, Washington State
Mallory Fox, Washington State
Natalie Elkind, Washington State
Kasey Langdon, Oklahoma State
Siera Strawser, Oklahoma State
Yolanda Odenyo, Oklahoma State
Amanda Cinalli, Notre Dame
Carrie Dew, Notre Dame
Jen Om, Princeton