I just took a peek over at OC. What I've learned is:
- Doom does not support the modem. You must use the BBA.
- Doom does not support static IP. Your router must be able to assign your DC an IP address using DHCP.
- Doom does not have a server. You must directly enter the IP addresses of the people you want to play with.
If you're playing on a LAN, this is relatively easy. If you want to play over the Internet, this is harder. You need to exchange your public IP with the other players ahead of time, and if you have a dynamic IP, you'll have to do this over and over each time your IP changes. Each player will also need to configure port forwarding on his router in order to receive the incoming connections from other players. Since OC doesn't specify the ports that Doom uses, you'd have to put your DC into a DMZ.
AFAIK all other online games have central servers and do not require port forwarding for direct connections, so playing Doom is a PITA in comparison to most online DC games, and since you need a BBA to do it, most people won't be able to play it at all.