Prado, 30, this season is batting .270/.317/.370 (91 OPS+) in 106 games for Arizona. For his career, he owns an OPS+ of 107 across parts of nine major-league seasons. He's signed through 2016.
O'Brien, meantime, is a career .266/.318/.533 hitter across three minor-league seasons. The 24-year-old O'Brien reached Double-A this season.