MESA, Ariz. -- Only a few roster spots are up for grabs in A's camp, and the competition is almost exclusively limited to pitchers, save for a possible extra outfielder.
These storylines have already dominated the early part of Spring Training: Who will fill out the rotation? Who will be bumped from the bullpen?
Decisions likely won't come down until the conclusion of camp, but the advent of game action -- the A's open Cactus League play at home on Friday against the Angels -- will only bring more fervor to these conversations.
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Injuries and other unforeseen developments could change the dynamic of these discussions. Of course, performance will largely dictate them, and the main characters could have company, should the less-talked-about contenders make some noise.
Here are a few dark-horse candidates who could land on the A's Opening Day roster.

A.J. Puk 
The A's maintain that they'd like to have their top pitching prospect start the season at Triple-A Nashville, but they could be tempted to keep the lefty in the Majors when breaking camp.
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Puk, who has never pitched above Double-A, is already impressing, and the A's are eagerly waiting to see how he responds in game action. They already have a plethora of candidates for their three open rotation spots, but you could hardly blame them should they at least entertain the thought of adding the 6-foot-7 Puk to this mix.
Lou Trivino 
The hard-throwing Trivino pitched 35 innings with Nashville last year, and he was added to the club's 40-man roster ahead of the Rule 5 Draft. Trivino brings elite velocity -- reaching triple-digits on the radar -- and excellent movement, both on display in front of the big league staff during workouts this week.
The A's bullpen is already overflowing, making Trivino's chances of making the team unlikely, but it doesn't seem impossible for him to push himself onto the 25-man roster.

Dustin Garneau 
The A's fully expect to have primary catcher Bruce Maxwell on hand when the regular season begins, but there's an outside chance that his ongoing legal issues force them to pivot to Garneau as a platoon partner for Josh Phegley. The A's liked what they saw from the backup catcher during the final months of play last year, getting strong defense behind the plate.