Hermann, this is one of the ways that OmniOutliner differs from a spreadsheet application. In an outline, parent rows appear above the child rows. Click on the parent and the descendants appear beneath. There's some overlap between OmniOutliner and an app like Numbers, but OmniOutliner isn't meant to completely replace a spreadsheet.
It's possible that we have a feature request open for a "bottom up" manner of organizing your outline; wouldn't hurt to email the request to the
support ninjas in case other folks need this as well. (All other things being equal, more popular requests get assigned higher priority.)