[Armanini]

Hi, I had the very same issue some time ago. You can have a look at a post called "Copy Column" (or something similar). Omni developers posted an easy AppleScript there, which allows you to copy single columns into a new OO file. By way of exporting single columns from the newly created OO files you'll be able to retain all your information and import it again into a different format. It's only a workaround, maybe a little slow, but it worked for me.