Oakland selected Armenteros' contract from Triple-A Las Vegas on Friday, Martin Gallegos of MLB.com reports.
Armenteros is now part of the Athletics' 40-man roster and will be protected from next month's Rule 5 Draft. The 24-year-old outfielder put up an encouraging .879 OPS with 20 homers and 17 steals in 110 games this past season between High-A Lansing and Double-A Midland.