Ok, I've created scores of VMs on Windows hosts, but now I'm into new territory. Just got a new laptop, purpose-built with Ubuntu. Installed Vbox. Created a VM running OL6. Easy-peasy, just like doing it in Windows. Installed the guest additions, with the following results:
[root@vblnx6-01 cdrom]# ./VBoxLinuxAdditions.run
Verifying archive integrity... All good.
Uncompressing VirtualBox 5.1.8 Guest Additions for Linux...........
VirtualBox Guest Additions installer
Removing installed version 5.1.8 of VirtualBox Guest Additions...
vboxadd-service.sh: Stopping VirtualBox Guest Addition service.
vboxadd.sh: Stopping VirtualBox Additions.
Copying additional installer modules ...
Installing additional modules ...
vboxadd.sh: Building Guest Additions kernel modules.
vboxadd.sh: Starting the VirtualBox Guest Additions.
Could not find the X.Org or XFree86 Window System, skipping.
[root@vblnx6-01 cdrom]#
Not sure what to do about line 13, but since this is a first-trial, decided to press on. Was able to mount a couple of host shared directories, exactly like I do with a windows host. No problems there.
Then, from the host desktop (Ununtu, running the Cinnamon desktop) connected to the VM with putty. Went to one of the mounted host share directories to unzip the installation media for Oracle dbatabase. Couldn't unzip it in place as 'oracle' because the directory permissions were 775, when on a Windows host they are 777. Pressing, on I created a 'download' directory in the root of the vm, owned by oracle:oinstall and unzipped there. Now things got totally unworkable. The Installer launched, but the first dialog screen looks like this:

As you can see, a lot of missing text. I'm guessing it might have to do with missing fonts, possibly related to the error produced when installing Guest Addtions, but I'm out of ideas on how to proceed.