Adapted script to copy a spreadsheet into OmniOutliner
 

I adapted the Create New Outline from Clipboard to handle tab-delimited text in the clipboard, which is what most spreadsheets generate. You could probably improve it to grab the first row and make column heads, but I figured it was simpler to ignore that and let the user copy and paste them as desired.

Doesn't address embedded newlines, formatting, merged cells or any of that. I don't think spreadsheets commonly output RTF data anyway. You'd be better off applying styles anyway.