You will need to download the OneDrive app to your iPad. You can either import them into OnSong directly from there. Or you can go into OnSong > Tap Songs > All Songs > "+" Sign > Import/Files > Select Location > OneDrive > Sign in > Select files > Import.
You will need the OneDrive app on your device.
Then go into OnSong > tap Share Menu > Export > Tap the Folder icon > Files > Select OnSong Backup > Export > It'll Switch over to a different screen, Locations: Select OneDrive and if you have a specific folder > Export.
***If OneDrive doesn’t show up in Files...
You will want to enable OneDrive in the Files app. In Files > Locations > Tap the circle icon with 3 dots in the middle > Edit > OneDrive will appear and you will want to toggle it on. Then you can go into OnSong and follow the instructions provided.
Note that this will work with pretty much any other cloud storage the same way (box.com, iCloud [just need the files app loaded], Mega, etc.)