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.)