The simply-named Oi, made by Australian cycling gear company Knog, is a bike bell for people who don't like bike bells. Such folks may be particularly keen on the latest model, the Oi Prima, as it's sleeker, slinkier and better-functioning than ever before.
These are smaller and some bars have limited space for accessories?
Personally, I’m gonna buy one for my Portola because the stock bell is nice and small, but it sucks: The hammer does not work reliably. This one has a similar footprint.