Someone mentioned the "tradition" ended in the 90s during the Blake beatings, another mentioned the Boomer/Big Red Sports incident. I think it started long before that as I recall when I was a student in the late 70s, early 80s, I don't recall doing a "Go Big Red" chant as fans. Of course we always yelled "Boomer" and "Sooner" from one side of the stadium to another, but no "Go Big Red" chant. I do remember seeing it printed on t-shirts and hats, but those seemed to fade away over time and now you never see anything with "Go Big Red" printed on it. If memory serves me correctly, there is a song that the OU band plays where we used to say at the end of the song, "Go Big Red," we now say "Go OU."
These days, I now associate "Go Big Red" with Nebraska and never use it in reference to OU. Truthfully, I don't miss it. I always thought it was kind of a misnomer anyway since our school colors were crimson and cream. Calling ourselves as "red" just didn't seem right, so if schools like Nebraska, or Cornell want to go by "Big Red," then I'm okay with that.