I need to know whether or not SharePoint will accept updates when changing the Oracle entry. Does SharePoint have this feature out of the box, or do I need to enable/create a solution? I would try this, but I do not have access to the Oracle database.
It depends on how it is uploaded to SharePoint. If it is just uploaded into a document library, then you may have to check it out in order to update it and you will have to click to refresh the data from Excel -- in other words it should work just as it does from the desktop.
If you plan to use Excel Services, then you'll need to be sure you are using Kerberos authentication to make it behave correctly. If not, the data will load the first time, but then refreshes won't work because of what is called the "double hop" problem. You will still likely have to push a button to refresh the data. Excel doesn't seem to want to refresh automatically from SharePoint.