Purushottam Nagesh Oak (2 March 1917 – 4 December 2007), commonly referred to as P. N. Oak, was an Indian writer, notable for his Hindu-centric brand of historical revisionism. Oak's "Institute for Rewriting Indian History" issued a quarterly periodical called Itihas Patrika in the 1980s.
Oak's claims, e.g. that Christianity and Islam are both derivatives of Hinduism, or that Vatican City, Kaaba, Westminster Abbey and the Taj Mahal were once Hindu temples to Shiva, and their reception in Indian popular culture have been noted by observers of contemporary Indian society. In addition to this Oak again asserted that the Vatican was allegedly originally a Vedic creation called Vatika and that the Papacy was also originally a Vedic Priesthood. He wrote books in three languages.