Oracle IsolationLevel

I'm trying to set the IsolationLevel property of the TADOConnection component to ilRepeatableRead or ilSerializable for an Oracle database and can't seem to get it to work. I built a little test app to test this. Whatever value I set it to, it just acts like ReadCommitted. Changes committed by others are visible to this app during the transaction. Except for the connection string all other properties are set to their default values. I'm reading the database with a simple select statement in a TADOQuery. What am I missing? (Delphi 5 - upgrading soon I hope!!)
.