Adrianna Franch made the only save during the free-kick round when she went low and to her right to stop OU's Jordan White.
"Ever since we lost to Rutgers my freshman year we've been practicing and practicing our PKs," Lopez said. "Before I walked up I knew I was going left."
It's only the second time penalty kicks have been needed to decide the final game. Texas won 5-4 on kicks against Colorado following a 1-1 game four years ago.
Second-seeded Oklahoma State (17-3-2, 8-2-0 and ranked 11th nationally) extended its all-time series lead over the Sooners to 15-2-1, including wins in the past eight contests.
"They had us on our back foot the first 15 minutes or so, but after that we found our way," OSU coach Colin Carmichael said. "It was a very tight game, and we expected that.
"Krista (Lopez, the game-winner) stepped up. This was her moment, and she made the most of it."
Fourth-seeded Oklahoma (12-8-2, 5-4-1) will wait until Monday to find out if the Sooners get a bid into the NCAA Tournament. OSU has earned an automatic bid.
"I think we'll get in -- absolutely," OU head coach Nicole Nelson said. "We made a clear case for an at-large bid."
Oklahoma is unranked, but the Sooners defeated ranked Texas (No. 21) and Texas A&M (No. 6) in San Antonio.
The Cowgirls became the first team in this championship to figure out Oklahoma goalkeeper Kelsey Devonshire. A sophomore from North Richland Hills, Texas, Devonshire was in goal for shutouts over Texas and Texas A&M in the Sooners' first two games of the event.
She went almost 250 minutes in the championship before one got past her. When Kyndall Treadwell scored with 20 minutes left in the game, it tied the score 1-1.
Devonshire came out of goal and went after a ball past the penalty box, but she collided with one of her own defenders. It left Treadwell, who scored the game-winner in the semifinals against Nebraska, to gather up a loose ball and kick it into an open net.
"A little breakdown in communication, but we continued to fight," Nelson said. "Even in the PKs, Adrianna (Franch) made a great save against us."
The Sooners got on the scoreboard just two minutes into the game when it appeared the Cowgirls fell asleep with the ball deep in their own zone.
A loose ball was heading toward the end line with only OU forward Caitlin Mooney in pursuit. She hustled it down and passed back in front of the goal to Annalisa Hall about five yards in front of the goal. Hall had time to foot it in low and to the right of Franch toward the post for a surprising 1-0 OU lead.
"Coming out at the beginning for OU that's the way you want to start a game," Franch said. "We weren't awake in the beginning."
Hall, a freshman midfielder from Friendswood, Texas, scored her first collegiate goal.
"We fell asleep a little bit and came out flat," Carmichael said. "Credit OU. They capitalized."
GK Adrianna Franch, Oklahoma State
D Melinda Mercado, Oklahoma State
D Lauren Alkek, Oklahoma
D Alyssa Mautz, Texas A&M
MF Dria Hampton, Oklahoma
MF Rachel Shipley, Texas A&M
MF Kristen Kelly, Oklahoma State
F Morgan Marlborough, Nebraska
F Krista Lopez, Oklahoma State
F Whitney Palmer, Oklahoma
F Jordan Jackson, Nebraska
Offensive Most Valuable Player
Morgan Marlborough, Nebraska
Defensive Most Valuable Player
Melinda Mercado, Oklahoma State