The Oi bell is different from other bells, it's circular and fits around the handlebars (it's easier just to look at the picture).
The bell materials available are aluminium, brass, copper, and black (though only copper and brass seem to be available at the moment) and it was also available in Titanium via Kickstarter.
It comes in two sizes, small and large - fitting 22.2mm and 23.8 to 31.8mm handlebars respectively.
The chime is quite pleasant and the design is definitely different.
It's available to buy on-line for €19.99 through the Knog site