You don't say how secure the file has to be?
The simplest option is to work online on a single file, which can be downloaded as required for offline use. I'd go with Google Drive, personally, with google Sheets online. Failing that, if you want web service on a secure basis you end up paying to someone like box.com.
But, the simplest way if you don't want to work online is probably just to use
FreeFileSynch to synch two local files, one in the One Drive folder and one in the doc folder, Dropbox is another alternative.