Oldham captain Korey Smith has joined Bristol City on a three-year deal, while Liam Kelly has left Ashton Gate to join the Latics.
City have paid an undisclosed fee for 23-year-old Smith, who had a year left to run on his deal at Boundary Park.
Fellow midfielder Kelly has also committed to a three-year deal, after an injury-hit spell at City.
The 24-year-old arrived from Kilmarnock in January 2013 but a knee problem restricted him to 21 appearances.
Smith came through the academy system at Norwich and had loan spells with Barnsley and Yeovil before arriving at Boundary Park in March 2013.
The midfielder signed a permanent contract last summer and he made 52 appearances, scoring twice, last term.