I would also like to have this feature and would definitely use it. Strangely, I just googled to find if footnotes were possible in OmniOutliner and this post was third on the list. This feature request is very relevant to me. I do a lot of early drafting of sections of my dissertation in OO and would like to insert footnotes for my many references. I like the freedom of OO that allows me to think and write without the anxiety that Scrivener and Word give me. Both these applications have a strong air of finality about them.