The concept of Bion of O is both essential and elusive like few others. By its ambiguous nature it gives rise to the most disconcerting interpretations and many misunderstandings. Among these is the idea of inaugurating a “mystical” psychoanalysis. To refute this thesis, the author proposes a path of reading that is certainly personal and at times even impervious, but that aims to insert this notion within a theoretical framework that is as clear, logical and coherent as possible. The meaning of the concept of O is only delineated in the relationship with other key concepts (mainly: transformation, invariant and at-one-ment). It therefore escapes us if we do not take into account the dialectical tension that arises from this fruitful trade and if we do not pay attention to the always different discursive contexts in which it is found. But above all it is important to realize that the concept of O is peculiar also in comparison to other more clearly specified and circumscribed concepts of Bion. In fact, it itself consists of a constellation of concepts, as many names or partial definitions.