O'Neill will join the squad, under the direction of head coach Tab Ramos, from December 15-22 in Sunrise, Florida, with a closed-door friendly match against Venezuela on December 21.
"We're excited that Shane's hard work has paid off with his selection to the Under-20 National Team," said Colorado Rapids Technical Director Paul Bravo.
"It's a great honor for him, but also for the club and our academy, and shows that young players in our system are being noticed on a national level."
O'Neill, who made his MLS debut in the 3-0 victory over Portland Timbers on September 5, had previously joined the U.S. U-20's for the Marbella Cup in Spain from October 6-17 of this year.
He earned a start in the 2-1 loss to Canada (playing 65 minutes) before an injury kept him out of the remaining two games of the tournament.
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