I had no problems with a VirtualBox 4.14 Windows 8 preview VM and x64 Oracle 11gR2.
My VM uses 40 GB Hard Disk. While installing it downloaded .Net Framework 3.51
I started with a fresh Windows 8 VM in VirtualBox 4.14 and installed guest additions and run Windows update once. Afterwards I installed Oracle and got the items below on my Start page.
I find the databasefiles of my new database Bk_W8 in C:\app\berndk\oradata\Bk_W8. That is the location I would use for additional tablespaces.
The rest can be done by scripts which can be executed by sqlplus from this machine or from another client and is not Windows 8 specific.
By right-clicking on the item on the start page, via advanced | open file location I found the path to the start menu:
C:\ProgramData\Microsoft\Windows\Start Menu\Programs\Oracle - OraDb11g_home1\Konfigurations- und Migrations-Tools
I guess you will find
the other is specific for a German Oracle installation.
To get access to the hidden C:\ProgramData folder see this link showing how to access folder options in Windows 8
Final hint: My VM currently uses 23 GB for the virtual hard disk.
OK I'm going to retry this with the Oracle Database Express Edition 11g Release 2 (September 2011). But that takes some time.
The is the Web interface of Oracle 11g XE in Windows 8
For the question of creating addition databaseinstances:
from this question on SO we learn, that it is not possible.
Creating tablespaces and users can be done using sqlplus. Which I found at C:\oraclexe\app\oracle\product\11.2.0\server\bin\sqlplus.exe.
Now going to remove that Virtual machine. I think there are better ways to learn Oracle.