Hefner, 25, was claimed by the Pirates from the Padres in November but designated for assignment by Pittsburgh on Friday. The 25-year-old posted a 4.98 ERA, 6.9 K/9, 3.5 BB/9, and 1.2 HR/9 in 157 1/3 innings at Triple-A this year. Baseball America ranked him 15th among Padres prospects prior to the season, noting that his "three average offerings and feel for the strike zone make him ideally suited to fit in the back of a rotation."
May, 27, hit .238/.330/.427 in 286 Triple-A plate appearances this year for the Diamondbacks' and Royals' affiliates. Olson, 28, is a former first-round draft pick who posted a 3.05 ERA, 6.4 K/9, 4.9 BB/9, and 0.7 HR/9 in 85 2/3 Triple-A innings for the Pirates' affiliate.