Based on the Kamen Rider OOO tv show (21st in the Kamen Rider series) - the OOO Driver Belt and O Scanner is responsible for transforming the protagonist into Kamen Rider OOO.
If you're not familiar with the Kamen Rider series, watch the video below to see how the belt works.
Kamen Rider OOO Papercraft - OOO Driver Belt [Download]
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