How can I make OneDrive the default backup in my iPhone, instead of iCloud?

You can't, but you could manually copy the backup file and paste it over in OneDrive, or any external drive for that matter.

I would recommend you create temporary space on your Laptop. Take the iPhone Backup and move it to the external hard drive thus clearing the temporary storage.

For iPhone backup using iTunes (Transfer to external Hard Disk)

1. Open Finder > Go > Go to Folder... > [~/Library/Application Support/MobileSync/]
2. Open Backup Folder
3. Copy Paste the BackUp file (Entire Folder) to any External Hard Disk