Using iPhoto with an already existing folder I have a pretty big (~60G) already existing folder of photos residing in /Users/ME/OndeDrive/Pictures . I want to be able to use iPhotos with that folder without (a) changing its structure, (b) creating copies of all the files (which is what is done if I do "Import to library"). Is this possible?
You can create a referenced library.

Hold ⌥ when launching iPhoto and choose Create New, then select a location for storing the library. This library won't hold the masters but will hold versions and metadata.

Upon opening this library, go to Preferences → Advanced and uncheck Copy items to the iPhoto library. This will make any new imported photos stay in their original location and only be referenced in the library.