I would treat these as two separate projects. 1) getting everything currently on the computer to onedrive and 2) ensuring all new files get posted.
For the first project, you can use PAD to "Get all Files" and this will even look through to subfolders. Once you have the list of files, you could say, for each file in Files, save to OneDrive.
The second project is hard; however, if you are looking for a PAD solution, you should be able to use the same logic:
- Get all files
- For each file in Files
- If the Last Modified Date is > The Last Time we ran this flow
- Save file to OneDrive
- EndIF
- End (For Each)
Best of Luck!