It was the second off-the-field incident for Underwood, who didn't play in an NFL game last season. He pleaded no contest to a misdemeanor count of soliciting a prostitute in June 2010 at a resort cabin in Lake Delton, Wis.
Underwood, who also was slowed by a knee injury during last year's training camp, is the first former Packer to head to Oakland under new general manager Reggie McKenzie, who spent 18 years in Green Bay. Underwood played cornerback in his first two NFL seasons, then switched to safety.
The Associated Press contributed to this report.