There is a track changes equivalent, which interoperates fine with Word (it's in Edit/Changes in OO). It also has a 'Note' facility which I assume is the same as Add comments in Word, although I've never used this.
In terms of general compatability with Word, it's fairly good, but not perfect, but in my experience no worse than the differences between different Word versions. And yes, Ubuntu and almost any other major Linux distribution will come with OO installed, or extremely easily available.