This study aims to investigate the relationship between the phallus concept and the object a included in the logic of fantasy, as a structure which suggests the relationships established between subject and object. To this end, we decided to draw on the Freudian Oedipus complex, with the contributions from Lacan on the paternal metaphor, to situate the place of the phallus in the subjective constitution. Next, we attempted to investigate in Lacan, with what he retakes on the contributions of Freud’s work, the concept of object a and the ramifications of this theoretical invention which refocuses the concept of the phallus, of castration and, consequently of Oedipus himself. Finally, in defining what the concept of fantasy gives to the possible relationships between subject and object, we attempted to extract some clinical consequences on this response which is built facing the enigma of the Other`s desire.