O'Brien, of St. Paul, Minnesota, had spent his entire childhood playing different sports, including football, but then he got a pain in his left knee that would not go away, he said during a speech, according to ESPN.
In December 2013, when he was a high school freshman, O'Brien was diagnosed with osteosarcoma, an extremely rare form of bone cancer.
In all, O'Brien underwent chemotherapy and 14 surgeries, including a knee replacement.
On Saturday, though, he was able to prove that determination breeds success when he stepped foot on the field in Piscataway, New Jersey, as a Golden Gopher wearing No. 14. It was a moment the college sophomore had been waiting for.
During Saturday's game, he took the field for the first time and held the ball for an extra point in the fourth quarter.
"There's been so many ups and downs and nights in the hospital and surgeries ... that has gone into this moment. ... This is what I've dreamed about and tonight it got to come true," he told the Big Ten Network on Saturday.
Afterward, he got hugs from his team and then was seen giving an emotional embrace to University of Minnesota head coach P.J. Fleck.
"It was a special moment with Coach Fleck. I mean there's one coach in the country that took a chance and gave me a spot to walk on. And I just thanked him. ... He means the world to me," O'Brien told the Big Ten Network.
Fleck told the Big Ten Network after the game that "when you think courage, you think Casey O'Brien."
"He's an unbelievable person and he's been through an awful lot. ... No one can ever take away that he played college football in the Big Ten. ... It was pretty emotional," Fleck said.
The Gophers went on to seal their win against Rutgers, 42-7. After the game, O'Brien was seen running over to share the moment with his parents.
O'Brien shared some advice to others who watched him take the field against incredible odds.
"Don't give up because you can make it happen. ... Go Gophers!" he said.