How hard is it to change a rear oxygen sensor on a 2002 Subaru Forester?

That's a tough one to answer. Just changed the rear oxy sensor on my '00 and yours should be about the same. My local auto parts store lends a set of sockets for just this purpose and I do recommend them. Takes some patience and trying different angles. Mine took a little more force than I could exert with a 3/8" ratchet but once it broke loose it unscrewed the rest of the way with ease. It is an awkward job but not hard. Keep the end of the new one clean, including no fingerprints. Does that answer your question? ...Ken