I-35, Dallas to OKC
I'm sure you don't need this info now, but...There's several places right off the highway.
At exit 31A in Ardmore is Two Frogs Grill on the east side of l35, great food, more upscale, steaks, etc.
At exit 51, Arbuckle Mountain Fried Pies, sweet and savory, on the west side of I35.
At exit 72 in Paul's Valley on the east side of l35 is an old fashioned hamburger/shake place: Ballard's.
You might want to post this on the DFW board. Lots of great Hounds there who've probably ventured to OKC.
The problem is that with a 3 hour drive, over 2 hours of that will be a food wasteland. Being a college town, Norman will have great places but you probably won't want to eat 20 minutes after you get in the car.
McGeehee's Catfish Restaurant in Marietta, OK is pretty famous. My parents took us there as kids 30+ years ago. Beautiful setting but about 15 miles off i35. I just googled it - looks to get about a 4 rating.
Sanger, TX is about 20 miles south of the OK border. There's a Babe's Chicken Dinner House on the square, just a few blocks from i35. Babe's is a fun place - not gourmet by any means - but a place we take out-of-town visitors about once a year. The drive from Sanger to Dallas is about 40 miles.
Sorry I couldn't help more. I do think the DFW board would give you more ideas. I make the trip to Norman once every 2 years, but we don't stop to eat on the way. Here's the link for Babe's: