Thomas Tuchel’s side reached the last four after a hard-fought 2-1 victory over Norway in Miami, setting up a meeting with the reigning champions, who defeated Switzerland 3-1.
The match could be Messi’s first and final encounter against England, with the Argentine superstar nearing the end of his career.
O'Reilly, who may face Messi directly from left-back, called him the greatest player he has ever seen.
"I can't wait. It's a once-in-a-lifetime opportunity. He's coming towards the end of his career. For me personally, he's the best player to ever touch a football pitch. And yeah, I can't wait for the challenge," he told BBC Radio 5 Live.
Messi has been outstanding at the tournament, scoring eight goals to lead the Golden Boot race and surpassing Miroslav Klose’s previous World Cup scoring record.