There are a number of tools and extensions which can be used in combination with Opencast.
Opencast Annotation Tool
The Opencast Annotation Tool is a free and open-source annotation tool. The aim of this software is to provide teachers and learners with an easy to use tool to annotate videos.
Opencast Annotation Tool on GitHub
pyCA is a fully functional Opencast capture agent written in Python. It is free software, licensed under the terms of the GNU Lesser General Public License. PyCA can be run on almost any kind of devices: A regular PC equipped with capture cards, a server to capture network streams, small boards or embedded devices like Raspberry Pi, Beagleboard, …
pyCA on GitHub