Yes, though it will require a little poking around under the hood.
Assuming you already have a document with a number column...
1) In the document inspector, click the gear (Advanced) tab and make sure "Compress on disk" is not checked.
2) Close your document in OmniOutliner.
** You might want to copy your document and make these changes in that copy if you don't have a backup of your file already. This should be totally safe, but I'd hate to ruin your Friday. **
3) Browse to your document in the Finder and right-click on it. A context menu should appear. Choose "show package contents".
4) Open the contents.xml file that appears in TextEdit or your favorite XML editor.
5) Look for a line exactly like this:
6) A couple lines above that line should be the name of the column that you want to have lots of decimal places.
7) Change the line from step 5 so it looks like
Or you can add even more zeros. But you probably want to add the same number of zeros to each number. And don't change anything else about the file.
8) Save and close the contents.xml file.
9) Open the outer .oo3 file in OmniOutliner again, and now when you type numbers with more decimal places, they should display.