The 6-5, 249-pound Williams started three games of the 2007 season, but suffered a torn left Achilles tendon in the fourth game at Tulsa and missed the remainder of the campaign.
"We're excited to have John back for another season," said OU Head Coach Bob Stoops. "We've felt for some time that John is an excellent player. He was off to a great start before the injury. I know he's looking forward to making a strong contribution for us next season."
Williams, who has battled knee surgery and Achilles surgery, said he's looking forward to playing a full season.
"The latter part of my career here is going to be my most successful," he said. "Everything is a process. This is what I'm living right now. I've got no choice but to move ahead."
The NCAA's decision to grant him another year of eligibility was a stress reliever for Williams.
"I'm very relieved, very thankful and grateful," he said. "I really want a chance to finish what I started."
The Houston native is continuing rehabilitation efforts. He will not participate in spring practices, but could have the green light for full workouts as early as June.
"Right now the left one feels almost as good as the right, but I'm still not doing a lot," Williams said. "My rehab is all about strengthening the Achilles."
Williams redshirted in 2003, worked in a reserve role in 2004, suffered a season-ending knee injury in the 2005 opener then played in eight games in 2006.
He has 13 career games played with four starts. His career tackle total stands at 15, including five for loss and two sacks.
In the 2007 Miami game, he forced a fumble that was recovered and returned 61 yards for a touchdown by Reggie Smith.
To qualify for a sixth year of eligibility, a student-athlete must miss two seasons of competition due to extenuating circumstances, including injury. A season of competition can be regained due to injury as long as the injury occurred in the first half of the schedule, and the student-athlete did not compete in more than 30 percent of the contests.
In Oklahoma's case, the waiver application is submitted to the Big 12 Conference. A waiver granted by the Big 12 is then used in support of the application to the NCAA.