This chapter offers an overview of ideas about O. One writes about O, however, knowing the futility of trying to describe something that is ultimately indescribable. O stands for the absolute truth in and of any object; it is assumed that this cannot be known by any human being". In Wilfred Bion's lexicon, the "field" in this excerpt from Rumi's poem is "O". It provides an apt description of Bion's most mysterious idea, representing absolute truth and the state of mind necessary to apprehend it. Symington and Symington describe the metaphysical or religious meaning in Bion's idea of O as providing a fundamentally different foundation to psychoanalytic thought, which is foreign to the works of M. Klein or E. Freud. Bion's idea of O, though derived from Freud's theory of the unconscious, goes further by including the idea of a transcendent unknowable realm of primal proto-mental knowledge, vestiges, perhaps, of pre-natal experience.