That's a good question. I'm only vaguely familiar with OEM, but I know enough about it to be dangerous.
Firstly, you will need an AutoCAD OEM license that includes the AutoLISP interpreter, otherwise the runtime won't be very useful. I don't know much about that, but I mention it just in case you didn't consider that.
Since the runtime links to the AutoCAD editor, you will need to build a custom version linked to your OEM license. This is not difficult if you have Visual Studio. I think Fred Tomke posted some detailed instructions for building the entire project. Those instructions would be a good starting point.
To build the custom runtime, you will need the OpenDCL source code on your computer along with Visual Studio 2008 and your OEM libraries and headers. You can download the source code from SourceForge in various ways. I use the TortoiseSVN SubVersion client.
You will need all the source code installed, but you'll only need to build the specific runtime modules that you need, so most of the project can be ignored. Off the top of my head, I think the only changes you should need to make to the stock project are to set the include and library directories to point to your OEM libraries.
Once the runtime modules are built, you'll need to figure out how to load them. I would use registry based demand loading to load them automatically.
That should be a fairly accurate, albeit brief, overview of the steps to take.
