Aguilar signed a one-year contract Wednesday with the Seibu Lions in Japan.
Atlanta signed Aguilar to a minor-league contract Tuesday.
Aguilar was released by the Athletics on Sunday.
The Athletics will designate Aguilar for assignment Monday, Daniel Alvarez-Montes of ElExtraBase.com reports.
Aguilar will serve as Oakland's designated hitter and cleanup batter in Sunday's game against the Royals.
Aguilar went 2-for-4 with a two-run home run in a win over the Reds on Sunday.
Aguilar is absent from the Athletics' lineup for Thursday's game versus the Angels.
Aguilar went 2-for-3 with a solo homer and one run scored in Saturday's loss to the Rangers.
Aguilar is not in the starting lineup for Tuesday's contest against the Cubs.
Aguilar is not in the lineup for Sunday's game against the Mets.