Relative paths within OneDrive (links work regardless of PC or Excel Online)
Relative paths within OneDrive - my documents folder is stored in OneDrive on my PC, so all items are synced. If I have a spreadsheet with external links, then it should work in Excel Online just like in Excel 365 on Windows.
Ideally, DSN connections (e.g. MySQL db files), refresh of Power Query when opening in Excel Online etc would also work regardless of platform, but I realise that is tougher