LaRoche, the 27-year-old brother of Nationals first baseman Adam LaRoche, received an invitation to Spring Training and immediately represents an extra infield option for the A's, who are not guaranteed a healthy Adam Rosales (right foot) by season's start.
Though Rosales recently told MLB.com that he'd like to be 100 percent by March 15, the A's -- who are also watching shortstop Cliff Pennington as he comes back from shoulder surgery -- are playing the safe route.
LaRoche hit .206 with four home runs and 16 RBIs in 102 games with Pittsburgh last year. Fifty-two of his 58 starts were at third base; he also made appearances at second base and first base.
Fellow newcomer Hollingsworth, meanwhile, has pitched just two games above Class A. Just 23, he was selected by the Rockies in the fourth round of the 2008 First-Year Player Draft. The right-hander notched a 12-8 record and 3.31 ERA for Class A Modesto, striking out 153 against just 34 walks. He made a brief debut at Double-A Tulsa late last season, and for his career, he boasts a 24-24 record and 4.33 ERA.
Mortensen, a former supplemental-round draftee, has pitched in the Major Leagues in each of the last two seasons but made just one start with Oakland last year. He made 26 starts for Triple-A Sacramento, notching a 13-6 record and 4.25 ERA and striking out a career-high 112 batters.