Odl

Anyone know where/how I can get info on how to write ODL code?

Here are the official documentations:

Type Libraries and the Object Description Language

Microsoft Interface Definition Language

I found this too, but it is pretty brief:

Basics of an IDL file

This is older but a little longer:

Understanding Interface Definition Language: A Developer's Survival Guide

This is nearly as ancient but might also be useful:

Tame Visual Basic with IDL

Also see chapter two of:

Hardcore Visual Basic

You probably want info on creating TLBs for use with standard DLLs, but most of what is written on the subject is about the main uses: COM and RPC type libraries.

This is another thing that .Net tried hard to kill off, which may explain why so little is being written about it anymore.

In the MSDN Library that came with Visual Studio 6.0, there is an article by Bruce McKinney entitled "Stealing Code with Type Libraries". It can be found under Technical Articles >> Visual Tools >> Visual Basic >> Extending Visual Basic with C++ DLLs.

EDIT

See also the Creating a Type Library section of the Notes for Developing DLLs for use with Microsoft® Visual Basic® v5.0.

EDIT 2

Yet another excellent IDL tutorial: IDL For VB Tutorial

Great find! I was looking for that on the web but didn't locate it.