Rob "Applescript drips from my fingers" Trew apparently had need of this same functionality at some point in the past. I am happy to provide his script here. To use, simply select one or more columns in your document, then invoke the script. A textual representation of all of the selected columns will be put on the clipboard, tab-delimited and including the column titles, which should allow pasting the data into a spreadsheet, new OmniOutliner document, etc. Unfortunately, it is not possible to paste a column into an existing OmniOutliner document alongside the columns already present, though one could do it with yet another Applescript.
For details on how to install Applescripts, see
the FAQ.