In its early TV years, Hanna-Barbera had a strong business relationship with Kellogg's, such that several of their characters appeared on cereal boxes for a time, and they used some of Kellogg's in-house mascots on their programs.
OK's was Kellogg's attempt at replicating the success of Cheerios from General Mills. While Cheerios boasted having Rocky & Bullwinkle as commerical endorsers, Kellogg's turned to Yogi Bear after their initial in-house mascot for the product, a Scottish bagpiper, was a failure. OK's were taken off the shelves in 1962, and so Yogi began appearing on Corn Flakes boxes.
Anyway, here's Yogi extolling the virtues, with help from narrator Don Messick, of OK's.