I'm using some kind of "Windows Basic Optical Mouse" or something. It's been in the house for years, it's ancient, the scroll wheel is utterly borked, but the thing still tracks like a boss so I'm A-OK with that.
I have no way of viewing or setting my Dots Per Inch (DPI) that I know of, even when I (in Windows 8.1) go to Control Panel --> Devices and Printers [Mouse] --> Pointer Options.
I've just set the pointer speed to the maximum available and turned off pointer precision (probably for the best, but it hurt me a bit for a while).
Osu sensitivity is left at 1x by default to avoid the wonkyness I found at higher ingame sensitivity.
Hang on here's the specs I've found so far;
Basic Optical Mouse
Manufacturer: Microsoft Corporation
Model: Basic Optical Mouse
Model Number: Unavailable
Well that's helpful. Not.
So I have no idea what it's real specifications are, when it was made, etc etc but everything appears to be in order.