No you can't replace your radio and have Onstar still work. Check out this link for more info
http://service.gm.com./gmtechlink/arcv_pdf/9_01_e.pdf
I know of some people that have added and aftermarket radio but also used a special harness to relocate the OEM one under the rear seats so the functions of the OEM radio ie Onstar and door chimes would still work.
Just a question if you already have the Bose system why would you want to change that is a fantastic radio.
Hope this helps
Leopardmech