I’m not quite sure whether I understand your “question”.
Do you want to run openSUSE from the Live-DVD media stored on a USB drive/external disk? Or do you want to install to an external USB disk.
The computer has no HDD of it’s own, or the internal disk cannot be used?
It is very well possible to install to an external disk/USB drive, leaving any internal disk untouched. Connect the USB-drive, boot from the LiveDVD, follow install instructions, but …at the partitioning stage, change the suggested partitioning. Create your “/”, “/home” and “swap” partition on the external disk, at the bootloader stage, make sure it gets installed on the external disk, remove any entries pointing to other than the openSUSE install on the external disk. That’s basically it.
----if i get this right i will have to make a /, /home, and swap on USB
----if the USB is bootable do not opensuse write on it too
----then i would not need a external HDD but only if i need to save larger files i.e. software or photos