If you have files on a PC computer that you would like to import into OnSong without using a cloud storage service, look no further than iTunes File Sharing. iTunes File Sharing lets your transfer files between an app running on your device and your computer. To do this, you just need to transfer files to your iOS/iPadOS device using iTunes for Windows in a file format that OnSong supports.
After you've transferred the file to your device, open the Songs Menu in OnSong and then tap on the + button to view the Add Songs menu. You can then choose iTunes File Sharing to locate the files, and select them for import.