So I did my last test last night:
After a factory restore, I logged into my Amazon account and did not download any books onto my Oasis. I turned wifi off and put the device to sleep at midnight - battery was on 17%
Turned it on again at 8:20 this morning and the battery was on 13%
So, dropped 4% in about 8 hours with no books loaded and wifi off
I'm going to contact Amazon now and see if they can exchange my unit