I am presuming you are using a Router and getting your local IP Address through DHCP (or you have assigned a local IP Address to your Server). Your Router is set to do Port Forwarding to your Server's local IP Address.
To test locally, your local DNS server needs to lookup directly to your local IP Address. You can use Network Utility "Lookup" function to check this, and check both forward (name-> IP) and reverse (IP->name) lookup. Then the DNS strings on you local nodes need to pointy FIRST to your local DNS Server, which then points on to an Internet-based DNS Server such as your ISP's DNS Server to OpenDNS.
My DHCP is set to pass out addresses starting at 192.168.0.101
mydomain.com looks up to 192.168.0.12, the Server Address. (also where the DNS Server lives).
My local DNS Server forward and backward looks up to that address.
My local Workstations all use 192.168.0.12 as their first DNS Address, then forward to Open DNS at 208.67.222.222 and 208.67 220.220
If you are not using DHCP, you need to be using a FireWall of some description. And you need to open the ports in that firewall to allow your web traffic to come in.