Hi Myriam,
From the FAQ which you can find
here and which I recommend for some reading:
Q.Can I import my OmniOutliner documents?
A.Yes, OmniFocus can import any OmniOutliner document. Note that OmniFocus can't directly display everything OmniOutliner can display (e.g., no extra columns with checkboxes or popups or calculations, and formatted text is only allowed in notes, not in other text columns). We try to preserve as much information as possible, copying that extra information into formatted notes.
If you want to Export to OmniOutliner there is a 3rd party Applescript written by one of our users available here:
http://forums.omnigroup.com/showthread.php?t=6357