The person spoke on condition of anonymity Tuesday because the agreement was subject to a physical.
The 31-year-old Callaspo played second base, first base and third base this year for Oakland. He hit .223, well below his .267 career average. A year earlier, he batted .258 with 10 homers for the Athletics and Los Angeles Angels.
Atlanta traded second baseman Tommy La Stella to the Cubs for right-handed reliever Luis Vizcaino last month. Callaspo and Phil Gosselin could compete for the starting job at second base with top prospect Jose Peraza.
Callaspo also could play behind Chris Johnson at third base.
Callaspo's best season at the plate came in 2009, when he hit .300 with 11 homers and 73 RBIs for Kansas City.
AP Sports Writer Ronald Blum in San Diego contributed to this report.