We did have religious movements with charismatic heads before television, you know. They reached the American public through radio and book publishing. Before radio, there were tent revivals. I think Oral Roberts would have mastered the mass media of a different era as well as he did the one of television.
Oral Roberts could have acheived the same level of success on other mediums, due to his charisma. He did not need to be seen to have the effect that he had. He could have succeeded in a traditional ministry in times when religion was practiced more often, or he could have succeeded with a radio program.
Without the invention of the television Oral Roberts could not have reached such a large audience and would not have gathered a huge following. New technology has improved upon the way we connect with people. In Oral Roberts case, it granted him access to thousands of people that wanted to hear his message. This greatly attributed to his success.
I think that Oral Roberts success was specifically with the dawning of a new era and the new trend of televangelism. While it is true that there are many mega churches (particularly in the bible belt) that are successful, they could have never had the "congregation" that Oral Roberts had at the height of his success.
Oral Roberts could not have reached the millions of people he reached without television. His weekly sermons would not have reached more people than he could fit in a large hall unless he had radio broadcasts and those did not draw people the way television did. Appealing to people face to face on television makes it more personal.