Phillips played some in reserve duty last season, and is considered a raw talent with a high ceiling.
Junior-college transfer Quincy Russell, from Trinity Valley Community College in Texas, was supposed to enroll early and be available for spring football, but academics pushed his arrival date back.
He's now expected to join the team in the summer, along with Oklahoma's three incoming freshman defensive tackles.
Like the Stoops brothers and co-offensive coordinator Jay Norvell, Montgomery played his college football at Iowa, where he was a four-year starting defensive tackle from 1998-2002.
“They sold me that I was a big piece of the puzzle and I was excited about that,” Montgomery told Sooner Sports TV. “I was excited to get here and get running.”
Montgomery replaces longtime OU assistant Jackie Shipp, who became Arizona State's defensive line coach Tuesday.
Shipp and offensive line coaches James Patton and Bruce Kittle were fired earlier this month. Stoops still has one opening to fill on his offensive coaching staff.
At Oklahoma, Shipp only coached defensive tackles, while Bobby Jack Wright handled defensive ends. It remains to be seen if Stoops will shuffle his staff and put Montgomery in charge of the entire defensive line.
“Just high energy and an enthusiastic coach,” Montgomery said when asked what he brings to the program. “Recruit my butt off and coach my butt off and represent OU.”