Click HERE for video highlights of the USA's tie with North Korea.
The game, in which the USA took 21 shots and the North Koreans unleashed 18, was played in steady rain.
Early in the second half, Kristine Lilly brought a ball down at the top of the penalty area passed to Wambach, who hammered a 13-yard shot off the gloves of diving North Korean goalkeeper Jon Myong Hui.
After Wambach clashed heads with Ri Kum Suk, she left the field for treatment, giving the Koreans an 11 vs. 10 advantage.
In the 58th minutes Kil Son Hui's shot slipped through U.S. goalkeeper Hope Solo's hands and dropped into the net. Four minutes later Kim Yong Ae scored from eight yards out.
Wambach reentered the field two minutes later and in the 69th minute O'Reilly struck from nine yards out high into the upper right corner.
"North Korea is by far one of the top five teams in the world and they showed that tonight," said Wambach. "They are very organized and very disciplined and very physical. They were probably more physical than we expected. With the surface and the rain tonight [the game] is a coin toss in a lot of ways, and you saw that."
The USA will plays Sweden on Friday (5 am ET live on ESPN). Sweden drew 1-1 with Nigeria in its opener.
Sept. 11 in Chengdu, China
USA 2 North Korea 2
Goals: Wambach (Lilly) 50, O'Reilly 69; Kil Son Hui 58, Kim Yong Ae 62.
USA -Solo, Whitehill, Rampone, Markgraf, Chalupny, Boxx, Lloyd, Lopez, O'Reilly (Kai, 92+), Lilly, Wambach.
North Korea -- Jon Myong Hui, 3-Om Jong Ran, Song Jong Sun, Sonu Kyong Sun, Kong Hye Ok, Kim Kyong Hwa, Ho Sun Hui (Kim Yong Ae (Jong Pok Sim, 90), Ri Un Suk, Ri Un Gyong, Kil Son Hui, Ri Kum Suk.
Yellow cards: USA -- Rampone 45; Jong Pok Sim 91+.
Referee: Nicole Petignat (SUI)