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Sanchez, 34, pitched in 12 games for the Royals in 2012 but was traded to Colorado for Jeremy Guthrie after posting a 7.76 ERA. He last played for Pittsburgh in 2013.
Sanchez had an 18.69 ERA in five Spring Training games.
O'Hearn, who homered and doubled in the Royals' 10-6 loss to the Reds on Monday, is a rising prospect, the organization's No. 8 prospect, according to MLB Pipeline.
The Royals now have 38 players remaining in camp.
Jeffrey Flanagan has covered the Royals since 1991, and for MLB.com since 2015. Follow him on Twitter @FlannyMLB. This story was not subject to the approval of Major League Baseball or its clubs.