Mkv is the container and with ogv you mean the Theora codec.
It is about what it inside the mkv container that you care about.
If you cannot decode it you cannot encode it to something else.
Generic advice: install the right codecs and be done with it.
If you really want you could convert the video (via ffmpeg) to VP9 (encapsulated as mkv or webm).
Specific advice: Sintel is a free movie and available in all kinds of formats so don't bother converting it: