Hi there,
From what I could understand is that you need to give access to your Oracle database server via the Internet and your network is protected by a firewall.
Firstly, you need to identify how your user wants to access the server. Is the user accessing it via a "telnet" shell or a "secure shell/ssh" or via an Oracle client?
Once the login procedure is identified, you can set a temporary access from your firewall by;
a) Open port 22 (for ssh) to allow connections from the Internet to the Oracle database server or
b) Open port 23 (for telnet) to allow connections from the Internet to the Oracle database server or
c) Setup a dial-in server with very strong authentication methods like having a call-back feature or by exchanging public keyrings (PGP way).
d) Use a VPN.
Hopefully this info helps.