Single user by default will be root, however you can switch to any other user.
when you see the boot screen for grub, press the 'e' key once
then using the arrows, choose the kernel you want to boot
then using the arrows, choose the like that start with kernel
press 'e' again, and add to the end of that line, '1' that will start the OS in init level 1, as you said single user, you can add 's' at the end.
then 'b' to boot that kernel
once the command prompt came, you can do su - oracle or the user you want
take note, there's no firewall, ssh or network, nor services on this run level.
Why you want to get into single user?
as ed point, you should try to follow the instruction and see what happen, it won't break anything.
I did some docs with screenshots for you, see them at:
Same doc, Word version and PDF