McCarron will back up starter Derek Carr.
The Raiders will be McCarron's third team after the 27-year-old spent most of his career backing up Andy Dalton in Cincinnati. The Bills signed McCarron in March to a two-year, $10 million deal after trading Tyrod Taylor to the Cleveland Browns.
The Bills have yet to announce their starter. Their choice will come between Nathan Peterman or rookie Josh Allen.
The Raiders released quarterbacks EJ Manuel and Connor Cook to make room for McCarron.