Saving documents to OneDrive or other online sites makes them available from other devices and easier to share.
With a document open in an Office app like Word or Excel, select File > Save (or Save a Copy). Then choose either OneDrive or Sites, then the appropriate site or folder.
If you haven’t signed in, do that now by selecting Sign In.
If you are creating documents in Office for the web or Microsoft Teams, the doc is being saved online automatically. If you need to save a new copy or rename, select File > Save as and choose from the menu.