O'Hara was born in Fayetteville, Georgia to parents Dan and Karen O'Hara. She has a brother named Jerry and a sister named Erin. O'Hara has Irish heritage. O'Hara grew up in Peachtree City, Georgia and graduated from Starr's Mill High School in Fayette County where she played four years on the varsity soccer team and captained the team during her junior and senior years. O'Hara helped lead the Panthers to the 5A state title in 2006 with 20 goals and 16 assists. The team finished second in the state championships during her sophomore year. O'Hara was named Parade All-American as a junior and a senior and All-League, All-County and All-State all four years. In 2006, she was named the 2006 Atlanta Journal Constitution (AJC) Player of the Year and Gatorade Georgia State Player of the Year. She was also named NSCAA All-American.
A two-time Parade All-American coming into her freshman year at Stanford University, O'Hara led the Cardinal in scoring in 2006 with nine goals. She repeated that feat during her sophomore year, helping the Cardinal to the third round of the NCAA Tournament.
O'Hara represented the United States in various youth national teams from 2005 through 2010. She scored 24 goals in her 35 under-20 caps, the third-most ever for a U.S. player in the U-20 age group. She was a member of the fourth-place United States U-20 women's national soccer team that competed in the 2006 FIFA U-20 Women's World Championship in Russia. O'Hara scored two goals in the tournament: one against the Congo (for which game she was named FIFA's player of the match) and one against Germany. She was also the first player in the tournament to be ejected from a game, having picked up two yellow cards in the game against Argentina.