Hart signed with the Athletics as a non-roster invitee Tuesday, Mark Feinsand of MLB.com reports.
Hart is set to become a free agent after declining his outright assignment to Triple-A Syracuse, Justin Toscano of The Bergen Record reports.
Hart had his contract selected from Triple-A Syracuse on Saturday.
Hart was designated for assignment by the Mets on Sunday, Tim Healey of Newsday reports.
Hart has been optioned to Triple-A Syracuse on Thursday, vcbeur reports.
Hart was called up from Triple-A Syracuse by the Mets on Sunday.
Hart was claimed off waivers by the Mets on Saturday and optioned to Triple-A Syracuse.
Hart was designated for assignment Wednesday, Adam McCalvy of MLB.com reports.
The Brewers optioned Hart to Triple-A San Antonio on Thursday.
Hart was recalled from Triple-A San Antonio on Monday.