Brennan took a handful of snaps with the Raiders' second-team offense Sunday, completing 4 of 6 throws. He also spent time with quarterbacks coach Paul Hackett after practice before heading to his room for a crash course in Oakland's offense.
Brennan was Washington's sixth-round draft pick in 2008. He was cut by the team on Monday after the Redskins traded for John Beck.
The Raiders made the move out of necessity.
Backup quarterbacks Bruce Gradkowski (groin) and Charlie Frye (wrist) are questionable to play against Dallas, leaving only starter Jason Campbell and backup Kyle Boller as the healthy quarterbacks on the roster.
Oakland coach Tom Cable didn't rule out Gradkowski or Frye playing against the Cowboys but said the decision to sign Brennan was precautionary.
"That's why we did it, in case both of those guys are still out by the time we leave Wednesday," Cable said. "It will give us a chance to finish the game up with him."
Brennan developed a solid fan base in Washington despite never playing in a regular season game in two seasons. He spent 2009 on injured reserve after hip surgery, then had knee surgery in March, which set him further back.
The Raiders called Brennan four days after he was cut by the Redskins, had him in for a workout between practices Saturday, then had him in uniform for the evening practice.
Oakland didn't announce the move until Sunday.
"Basically I'm just trying to get somewhat of a gameplan for Thursday night just so I can be ready," he said. "I can at least look like I know what I'm doing out there and have a chance to move the ball and have some success with that third team. Obviously with some of the injuries the potential of that is really high. I'm just getting ready for it and hoping for the best."
Brennan was a record-setting quarterback at Hawaii and a two-time finalist for the Heisman Trophy. He broke 31 NCAA records and holds the NCAA mark of 131 touchdowns in just 38 games, and received the Sammy Baugh Award in 2006, given annually to the nation's top passer.
Some questioned whether his freewheeling style could work in the more regimented NFL, but Brennan thinks it will work in his favor under the current circumstances.
"I've always had an ability to see the field and make a play," Brennan said. "Sometimes it drives the coach nuts but when it works I end up getting a touchdown or something good ends up happening. I can kind of hopefully rely on that Thursday night and see the field and make some good plays and hopefully throw no interceptions and make minimal mistakes."
DL Richard Seymour returned to practice for the first time since Thursday. ... DE Jay Richardson (knee) and RB Darren McFadden (hamstring) were held out. ... WR Jonathan Holland was waived injured to make room on the roster for Brennan.