Do you have the Oracle client software installed on the server where Crystal Server is running? If not, it needs to be there.
What user ID do the Crystal Server services run under? The default is a local services ID. If you need to access resources over the network, this needs to be changed to use a network account. We use a network account that was specifically set up for this where the password doesn't expire. This account then needs to be given access to see the various servers on that it needs to access on the network. You can test your Oracle connection by logging in to the server with this account and using tnsping to contact Oracle.
-Dell