In the United States’ first four games of the 2015 FIFA Women’s World Cup, O’Hara did not see any playing time. O’Hara made her first start of the tournament in the quarter-final game against China PR. She was replaced by Christen Press in the 61st minute. O’Hara scored her first career international goal in the United States’ 2–0 victory over Germany in the semi-final. In the final against Japan, O’Hara entered the game in the 61st minute to replace Megan Rapinoe. The United States went on to defeat Japan 5–2, winning the first World Cup title since 1999 and the third overall World Cup title for the United States since the inaugural Women’s World Cup in 1991.

O’Hara played for club teams, the Peachtree City Lazers and AFC Lightning before playing for the U.S. U-16s in 2004 and then joining the U-17 youth women’s national team of that same year. She played on the Concorde Fire South ’88 Elite that went on to win the 2007 GA U19G State Cup and advance to the Semi Finals of Regionals.

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.

O’Hara was drafted third overall by FC Gold Pride at the 2010 WPS Draft. In addition to the close proximity of home stadium Pioneer Stadium to O’Hara’s alma mater Stanford University, O’Hara had previously worked with FC Gold Pride head coach Albertin Montoya when he served as an assistant coach at Stanford University in 2008.

In February 2008, O’Hara returned to the U-20 women’s national team to play in the U-20 Four Nations Tournament in Chile. Her last appearance for the U-20 team occurred in July 2008, at the 2008 CONCACAF Women’s U-20 Championship in Puebla, Mexico. O’Hara helped the U-20 team qualify for the 2008 FIFA U-20 Women’s World Cup in Chile. She did not play in the U-20 World Cup, instead remaining with her college team in its NCAA postseason campaign.

As a senior, she had one of the best seasons in Division I history, scoring 26 goals with 13 assists. O’Hara’s senior year ended in the 2009 College Cup, where the Cardinal lost to North Carolina. O’Hara received two yellow cards in the second half, ejecting her from the game, forcing the Cardinal to finish the game a woman down. The game ended with a score of 1–0, thus marking North Carolina’s twentieth National Championship. She finished her college career at Stanford with 57 goals and 32 assists, both school records at the time.

She was called into the senior national team’s training camp in December 2009 and attended the January 2010 training camp in the lead-up to the 2010 Algarve Cup. O’Hara earned her first senior national team cap in March 2010, coming in as a substitute during a friendly match against Mexico.

The team dominated the season finishing first during the regular season after defeating the Philadelphia Independence 4–1 with goals from O’Hara, Christine Sinclair and Marta. As the regular season champion, the team earned a direct route to the championship playoff game where they faced the Philadelphia Independence. During the final, FC Gold Pride defeated the Independence 4–0 to clinch the WPS Championship. Despite their successful season, the club ceased operations on November 16, 2010, due to not meeting the league’s financial reserve requirement.

After FC Gold Pride folded in November 2010, O’Hara was signed by the Boston Breakers. She scored 10 goals during her two seasons in the WPS playing primarily as an outside midfielder. On January 5, 2012, it was announced O’Hara would be going back to her hometown because she had signed with the Atlanta Beat. However, the league folded just before the 2012 season began.

After falling short of making the 21 player World Cup roster, O’Hara was called up to replace Lindsay Tarpley who tore her ACL in a send-off match against Japan on May 14, 2011. O’Hara earned just one cap at right midfield in the 2011 FIFA Women’s World Cup in the final group stage game against Sweden. The United States went on to win the silver medal in that tournament.

Throughout her national U-20s, collegiate, and club career, O’Hara has been one of the top young offensive players in the United States, but under head coach Pia Sundhage, O’Hara was converted to play outside back in 2012 after teammate Ali Krieger went down with an ACL injury in the 2012 CONCACAF Women’s Olympic Qualifying Tournament. Against Guatemala on January 22, 2012, in the Olympic Qualifiers, she made her first start at left back and registered three assists. O’Hara made her first start at right back against Costa Rica in the match that qualified the United States for the 2012 Summer Olympics in London.

On January 11, 2013, O’Hara joined Sky Blue FC in the new National Women’s Soccer League. Because the club’s head coach, Jim Gabarra, played O’Hara as a forward, she reverted to a role she filled with success in college.

O’Hara has appeared in multiple commercials and advertisements for Under Armour. In 2015, she appeared in television commercials and promotional materials promoting chocolate milk on behalf of the National Fluid Milk Processor Promotion Board.

On December 29, 2017, O’Hara was traded to Utah Royals FC. Due to a hamstring injury, O’Hara only appeared in 8 games for Utah in 2018.

O’Hara was one of many out LGBT athletes to compete in the 2019 FIFA Women’s World Cup in France. During her off-season, she resides with her partner in Washington, D.C.

In July 2020 O’Hara launched a podcast with sports website Just Women’s Sports. Website founder Haley Rosen had asked O’Hara to join the advisory board. O’Hara said that she instead asked to host their podcast because she’d “always thought hosting a podcast would be fun.” O’Hara says her goal is to generate “open, candid conversations” about the lives of athletes, particularly female athletes.