It was a really popular Empathy feature request, it even has a bounty on it, and today I just did it! The patches are available in bugzilla, apply on telepathy-gabble and empathy master.
Here is a screencast of OTR in action between Empathy and Pidgin:
I hacked it on my free time, let’s see if crowd funded projects really works. If you like that feature feel free to make a donation on the bounty: https://freedomsponsors.org/core/issue/333/telepathy-should-support-otr-encryption.