Ensured the games are up to date and patched in the correct order
Ensured the servers I join run the same version
Re-installed the game multiple times, including to my OS hard drive instead of my games drive
Ran installers, games and patches as administrator
Ran each game in turn prior to patching, and entered the correct key on the Securom prompt (appears only when running OA for the first time)
Reformatted my machine
Checked with my ISP they were not blocking ports
Factory reset my router
Forwarded the relevant ports in my router
Allowed all IGMP traffic through the router
Assigned my computer a static IP and added it to the DMZ in my router
Attempted to connect through another router and ISP (Still D-Link though, just a newer one)
Used network monitoring software to ensure the game network requests were reaching my local machine (they are)
Talked to Placebo to check my keys are in the GameSpy database (they are)
Tried the beta patch
Downloaded the securom diagnostics tool to confirm securom keys were present (detected only OA, not ArmA2)