Simple answer at this point is, there is no support for odt edit within the browser on disroot. You need to download the file to edit it.
We are experimenting with collabora, which will allow for such thing (also document edition with multiple users at the same time), but at this moment it is far from having it enabled to everyone. The reason is simple. Performance. From our quick and dirty checks last week, the performance is terrible, and in order to satisfy the demand from disrooters, it would probably require lots of resources and even then it will probably not perform as smooth as one would expect.
That said, we are planning to open a beta test somewhere next week (or week after) to get better overview on what exactly is required in terms of resources (hardware) we would have to invest to have collaborative office setup for disroot.