Obedience [obey]: doing what one is told to do; doing what God commands in his written Word; the evidence of genuine faith.
Occult: To do with ‘hidden’ things. Includes a range of psychic, magic, spiritistic, shamanistic and mediumistic practices forbidden in Scripture, specifically in Deuteronomy 18:10-12, and generally throughout the Scripture, right up to Revelation 22:15.
Omnipotent: all powerful.
Omnipresent: present everywhere.
Omniscient: knowing everything.
Original sin: The nature, guilt and condemnation passed on from Adam to all humans except Jesus Christ [Romans 5:12-21].
Orthodox: holding correct doctrines.
Orthodox churches: churches descendant from the ‘Eastern church’ which separated from the ‘Western’ church in the 11th century. The key causes of this separation were  papal authority in the western church, and  the filioque clause, which Western churches had added to the Nicene Creed.