Well, I've loaded data into Oracle from Excel spreadsheets a number of times, but I've done it using the PowerBuilder pipeline utility.
I know you can extract table data from SQL Server and save them into .CSV files. You can set up a PB pipeline with ODBC connectivity to read the files. Then you define your input (Oracle) table in PB(ie. columns, datatypes, scale) then run the pipeline.
The only downside is that you'd have to do one table at a time and you'd have to purchase PB (~$3000). Again, there's probably more streamlined and cheaper ways to go about this, but at least I know this method works.