I've added an .ocx to the toolbox in VS. Two .dll's were created: Interop.NNN.dll, AxInterop.NNN.dll.
What is each one? Are they both required?
The AxFoo.dll assembly contains an automatically generated class that's derived from the System.Windows.Forms.AxHost control. It is pretty simple, it has methods, properties and events, the same ones you have available in the .ocx, that simply call the Foo.dll interop library.
So, yes, you definitely need to deploy both assemblies.