After much hair-pulling and name-calling I've managed to track down the problem with the help of our Oracle DBA. It turns out this was not a VFP problem at all, but an ODBC problem. It appears VFP has real problems with Oracle LOB's. Turning off LOB's in the ODBC config fixes the following problems in VFP:

1. Invalid argument in OCI Call errors
2. Oracle long field types displaying trash characters in views