Annotations (interoperable with MS Word's annotations) are a standard feature of OO.o now, and have been for a couple of years at least -- I think they came with 2.0, and we're now on version 3.2.

I work over copy-edited manuscripts with Ace these days using change tracking and annotations -- they use Word, I use one of the Mac forks of OO.o (there are three of 'em, confusingly enough). I notice you couldn't find the feature back in 2007 -- nearly five years ago.