I tried Oracle 6.4. on Virtual Box and didn't have to go the raw device direction. Just using a simple linux partition.
After configuring the asm driver,
I was stocked at Instantiating disk: failed to.
It seems like the driver isn't loaded directly after configuration but after rebooting.
After this I could create the disk without any problems.
So, if you just did a oracle configure, you would have to reboot the machine so that the driver get's loaded.
Probably there are better ways to load the driver.