The expansion is actually a standalone game. I haven't had any issues with either CoH or CoH OF. As mopar man said, patch in the correct order and use the correct patches. Some patches are for CoH only and some are for both games.
Relicnews is one website where you can find a link to all the patches. It is a fansite for Relic games such as CoH, Dawn of War, Homeworld and others.