4th. The answer you seek is that they have the 4th easiest schedule next year based off of the previous season's records for their 2013 opponents. Oakland's strength of schedule (SOS) rating this year is .469 because their opponents have a combined record of 120-136-0 for the 2012 season.
Though it isn't bad to have one of the easier schedules based off of the previous season, it can be a little misleading. The reason their schedule is so easy is because most of the division, besides the Denver Broncos, were not very good this past year.
When you are in the worst division from the previous year your SOS for the next year is going to be considered easy due to having played the other teams that were not good in your own division twice for that year.
New Orleans Saints
St. Louis Rams
Green Bay Packers
San Francisco 49ers
New England Patriots
Tampa Bay Buccaneers
New York Jets
New York Giants
Kansas City Chiefs
Its pretty much inevitable that somebody is going to complain that the Broncos have the easiest SOS for 2013 but the reason for that is clear. They were the only team in the AFC West that did well this past season and the combined records of the other 3 teams in the division were not good.
Those records get counted twice when computing their score and inevitably make them have the easiest considered schedule for next year. That is not NFL bias towards Peyton Manning, its simply that the other AFCW teams (Including the Raiders of course) did not have good seasons last year.
Another reason SOS is misleading is because it does not allow the chance for teams to improve. The strength of next year's schedule is not really actually going to go according to this list because some teams will be better and some teams will be worse. Hopefully the Raiders show just how much better a team can get from one year to the next!