The Gunners won the Women's FA Cup in 2016 and finished third in WSL 1. O'Reilly, whose contract length has not been disclosed, told Kansas City's club website: "I will have conversations about my potential future in the NWSL when those conversations need to happen."
She made her international debut in March 2002 at the age of 17, is the second American to join a WSL club so far in January, following winger Crystal Dunn's move to Chelsea Ladies from Washington Spirit.