Heather joined the club in January 2017 after leaving FC Kansas City.
She arrived in north London having recently retired from international football, in a career which spanned 15 years and saw her amass 231 appearances for the USA, scoring 47 goals. Her first appearance came in March 2002 when she was still in high school.
O'Reilly is a three-time Olympic champion, having won the gold medal as part of the successful USWNT side at Athens 2004, Beijing 2008 and London 2012.
Heather was also a member of the USA team which won the 2015 World Cup.
She is widely-recognised as one of the most popular players in the United States, and is the eighth most-capped player in the history of the women's national team.
With a reputation for tireless professionalism, commitment and teamwork, Pedro Martinez Losa described her signing by saying: "She will bring here a legacy of values, hard work and professionalism which will also help our young players to develop. It is a privilege for us to be able to bring her to Arsenal Ladies."
Heather is due to make her first appearance in Arsenal colours at Kelly Smith's celebration match on February 19, 2017.