What's here[NB: this is a guided tour of an experimental service. It bounces around a lot, so if it's not working for you, try again. Frustration at repeated, unsuccessful attempts can be vented at the author, tom-dot-bruce[somewhere in the vicinity of]cornell-dot-edu]
This "guided tour" will walk you through the prototype OAI repository server at the Legal Information Institute. It includes a little bit of explanatory text, but
Here's a schema diagram. It won't print well, sadly. There's a PDF version that might be more readable: OAI-lite.pdf
All metadata standards are compromises between completeness and accuracy of representation (on one hand) and ease of formulation and implementation on the other. Gathering support for any particular standard or set of standards requires momentum that can easily be lost in debate over minutiae. There is a built-in tendency to make the perfect the enemy of the good, when what is need are functional standards that are designed not to stand in the way of their own refinement at some later time.