The game's connection to another player goes through Steam's servers and isn't hosted here in-house. While we have some things to suggest to improve performance, we can't guarantee the reliability of any network connections we don't control.
1. Check and make sure you don't have a virus or malware on your system.
2. Make sure Windows is up-to-date.
3. If you are using wifi for your connection, try a wired connection. The occasional network hiccups from wifi take longer to re-establish than most wired connections.
4. Disable your anti-virus and firewall programs. If you determine they are not at fault, you can go back later and turn them back on.
5. A lot of previous users reported that they were using a voice chat program while using OMD2. Try disabling the outside program and using the in-game voice support.
6. Disable any peer-to-peer file sharing programs you are running.
6.You can try disabling IPv6. (You can download a program from Microsoft directly to do this at this link.)
7. You can try a VPN, such as Hamachi, to establish a more direct network connection with your coop partner. (Please be advised that setting up a direct network connection with someone else can leave you open to malicious attacks and should not be done lightly.)