Can I Get Bluetooth on My A8
 

I have a 2005 A8 that I purchased used. Is there any way to get bluetooth to work through the handsfree system installed as OnStar?

Its already installed, if not you just need the telematics module.
You should know that the factory bluetooth doesn't like some phones, like certain black berry models. Ask the dealer for a print out of compatible blue tooth phones, it should be available right at their computer.