You can create an Oracle Data Redaction policy that redacts column data by showing only null values in the query result.

This feature enables you to use the DBMS_REDACT.NULLIFY function to hide all of the sensitive data in a table or view column and replace it with null values. You can set this function by using the function_type parameter of the DBMS_REDACT.ADD_POLICY or DBMS_REDACT.ALTER_POLICY procedure.

For example:

function_type => DBMS_REDACT.NULLIFY