Add subtitles to Ogg Video
I've used several subtitles editors on Linux: gnome-subtitles, Ksubtile and KDVDAuthor mostly. In all cases, I've kept my subtitles as .srt files and then added them to the DVD when authoring.
As for embedding subtitles in a video in ogv, this link should probably help you.
There is another related link: https://archive.flossmanuals.net/ogg-theora/subtitles/embedding-subtitles.html