Both the large size Oi Classic and Oi Luxe bells come with a spacer / shim to suit mounting to bars 23.8, 25.4, or 26mm. This part is found hiding in the underside of the tray that the bell sits in, so make sure you pull this out before recycling that packaging!
For further mounting instructions, please head here for the Oi Classic, and here for the Oi Luxe.