I suggest starting simple. Please try first to display data from Oracle in a PI table without any query parameters from a PI TreeView web part. Does that work?
If not, you should be looking into possible issues with your connection against Oracle. If this doesn't work please try using another client but the same provider on the same machine to execute a query. If this works, the issue can be security related i.e. the application pool identity not having appropriate rights to access tables / views in Oracle. Please check log files on the Oracle client and server for ORA-XXXXX messages pointing to the root cause of the problem.
Answers to the following questions could help to identify any known restrictions.
What's the Oracle server version?
What the Oracle client version installed?
What the exact name and version of the provider used?
Is this an OLEDB or an ODBC driver?