This reference is from the UK government.  Its proper name is Management of Risk: Guidance for Practitioners, by the Office of Government Commerce (OGC). The abbreviation M_o_R is established within the book.

The OGC is no longer the supplier and may not exist.  M_o_R can be bought from Axelos in print or download editions.  (Axelos is also the current owner of PRINCE2, ITIL, etc.)