Being pedantic, OLE1 was the predecessor. OLE2 used COM for its plumbing.

Wikipedia has an article on the file format [1]. It was quite nice. It works like an uncompressed zip file with transactional updates.

Earlier Word document formats were much worse. They were a dump of Word's memory contents. Saving and loading was very quick though!

[1]: https://en.wikipedia.org/wiki/Compound_File_Binary_Format