Documentation about .o and .a file format

Hi,
I'm currently writing a kind of linker for MinGW-C and C++ and BlitzMax compiler output (.o and .a files). Therefore I'm searching for documentation about the object file format which is used by MinGW and BlitzMax, which will be the input file format for my application.
Do you know where I can get documentation or do you yourself know how these files are structured?
All kinds of help are welcome.
Thanks in advance!

Any reason you're not just using ld (or any other free linker)? In any case you can get the sourcecode to that from the MinGW website, so that's probably a pretty good place to start. Otherwise hunt around on wikipedia.