[DerekM]

With OmniOutliner 3, changing a column summary or type is a destructive change. The only way to recover the previous values is by using undo. Setting the summary type to hidden removes what would be the summary cell. If you'e using 10.7 or 10.8, you can check if there's a saved version with the data you're missing by using the File > Revert Document option. Sorry for the confusion. If you have any questions about this, please let me know.