The move would give the team another backup option behind Carson Palmer, who is entering his first full year as the team's starter and hoping to improve after having to learn the playbook on the fly last season.
The former Heisman Trophy winner played in 12 games as a rookie, finishing with 2,547 passing yards to go along with 11 TDs, 12 INTs and a quarterback rating of just 74.0.
He lasted three more seasons in Arizona before being cut just prior to the 2010 regular season. He was signed shortly thereafter by the Houston Texans, where he served primarily as backup for two seasons.
Leinart looked to have his shot to resume a starting role in 2011 when Matt Schaub went down for the Texans, but he lasted just one half as Houston's starter before going down with a season-ending injury.
In Oakland, Leinart would reunite with Gregg Knapp, his quarterbacks coach with the Texans and the current offensive coordinator for Oakland.
The team also signed quarterback Rhett Bomar to a reserve/future contract earlier this offseason.