The following script will copy only the selected rows (no children, no notes, only the topic column) to the clipboard. The rows do not have to be at the same level in the outline.
Code:
tell application "OmniOutliner Professional"
tell front document
set theRows to get selected rows
end tell
set rowCount to (count of theRows)
if (rowCount is 0) then
display dialog "You must select one or more rows!" buttons "OK"
return
end if
set rowIndex to 1
set clipboardText to ""
repeat rowCount times
set theRow to item rowIndex of theRows
set clipboardText to clipboardText & topic of theRow & "
"
set rowIndex to rowIndex + 1
end repeat
set the clipboard to clipboardText
end tell