I'm not sure what you are doing with Oracle, or how that factors in here. Just get VirtualBox by itself, install it, and then point it to the Ubuntu .iso. Once it is installed, there are guides on Virtualbox's page and elsewhere online on how the command script to install the VirtualBox Additions, which will allow for larger resolutions in the guest OS.
Too many people keep messing with multiboot systems and bootloader issues, instead of just using current technology to virtualize other OSes. Keep your host system and primary OS clean!