Welcome to the forums, Olivier!
There's some information that may be of help to you in
this thread - the short version is that current versions of OmniFocus aren't really designed for this kind of workflow. It's possible to get this working after a fashion, but there are some fairly significant drawbacks. (This is something we're thinking about for future versions, though.)
It suddenly occurred to me that the Applying OmniFocus forum might be a better place for that thread to live than the OmniFocus for Mac forum. I'll move it over here...