The As will have at least one semi-surprise on their Opening Night roster: a minor-league free agent whose pitching resume is slim.

Reliever Jordan Weems, drafted as a catcher by the Red Sox in 2011, was told Sunday that hell open the season in the big leagues, The Chronicle has learned. Weems did not even pitch in high school, but he impressed the As thoroughly in spring 1.0 and during Camp Coliseum this month, throwing as hard as 98 mph and showing good control.

Weems really pitched outstanding, As pitching coach Scott Emerson said recently. He has a riding fastball, that split-finger fastball and an occasional breaking ball. For a converted guy to be able to command the top part of the zone with a fastball and something soft at the bottom of the zone is pretty good for a guy who hasnt done much pitching.

Catcher Jonah Heim, asked last week who has jumped out to him during Camp Coliseum, said, Weems has come out and looked real sharp. Throwing pretty hard it seems like. ... Weems looks really good.

Weems had an opt-out in his deal Saturday and likely would have had multiple big-league offers elsewhere had Oakland not placed him on the roster. The As can carry 30 players the first 15 days of the season, so the team is likely to try to maximize its roster flexibility by retaining players who are out of options or who have opt-out clauses. In addition, one of the As catchers, either Heim or Austin Allen, is likely to be on the up-to-three-man taxi-squad to start the season; in order to have a taxi squad, a team must include a catcher among the three.

Susan Slusser covers the As for The San Francisco Chronicle. Email: sslusser@sfchronicle.com Twitter: @susanslusser

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Hard-throwing reliever Jordan Weems to make A's roster - San Francisco Chronicle