Venditte can throw with either hand and Otero is a righty. Both previously appeared with Oakland this season.
Venditte made his major league debut on June 5, becoming the first ambidextrous pitcher to play in the majors since Greg Harris in 1995. Venditte made four appearances before going on the disabled list with a strained right shoulder.
The moves Saturday come after the A's went deep into their bullpen Friday night in a 13-inning loss to Baltimore.
Mills started for the A's on Friday night, his first appearance in the majors this season.