Oxford bags were first popular in the 1920s; modified versions were sold all the way up to the late 1930s. The story goes that students of Oxford University were not allowed to wear plus-fours (knickers in Yank speak) to classes. The students' rebellious solution was to wear flannel trousers with very wide legs over the plus-fours. After class, they would strip off the trousers and walk around in their plus-fours, happy as clams. These baggy, elephant-legged pants came to be known as "Oxford bags".
Rich young American males brought the fashion from England to the U.S. It never caught on with most folks, but the American menswear industry got wind of it and offered a modified version for about 10 years. The adapted "Oxford bag" found in Sears catalogues was called a "campus pant", "varsity pant", or "collegiate pant". Unlike the original Oxford bags, these ones' legs were not uniformly wide from thigh to cuff. Rather, they were slim along the thigh, then flared out from the knee down to the cuff. (That's right: '70s bell-bottoms were imitations of a '30s style!) The circumference of the leg bottoms were, at most, 24 inches -- about 4 inches wider than the average.
I have no Oxford bags in my collection, nor do I have photos of any. I did have a pair of "varsity pants", but sold them on eBay several months ago. Their leg bottoms weren't all that wide: about 22 inches, I think.