That's an overstatement, Oblivion neither invented the open world formula, nor did it popularize it except maybe on Xbox. It is the middle child between two better games - the awesome story and setting with rather poor combat that is Morrowind and the lean but shallow Skyrim. All that Oblivion managed to do is set a low standard for storytelling and side content in TES games, while still remaining pretty clunky in its own way. That and purchasable cosmetics of course, although it was hardly the first to do this either. Overall it was just a step on the path the industry has taken - neither the first, nor the most prominent.