Although O'Neill supposedly derived Mourning Becomes Electra from the Oresteia, the myth that actually structures the play's action is overwhelming that of Oedipus. Oedipus was the Theban king who unwittingly killed his father and murdered his mother, bringing ruin to the land. Famously Freud elaborated this myth into his Oedipus complex, the structure through which children are conventionally introduced into the social order and normative sexual relations.
At the center of this complex in what Freud defined as its positive form is the child's incestuous desire for the parent of the opposite sex, a desire possibly surmounted in the course of the child's development or else subject to repression. Its development is starkly differentiated for boys and girls. Both begin with a primary love object, the mother. The boy child only moves from the mother upon the threat of castration posed by his rival, the father. In other words, the boy fears that the father would cut his penis off if he continues to cling to the mother who rightfully belongs to her husband. By prohibiting incest and instituting the proper relations of desire within the household, the Father becomes a figure of the law. In surmounting his Oedipal desires, the boy would then abandon his mother as a love object and identify himself with his father.
In contrast, the girl abandons the mother upon realizing both the mother's castration and her own. To her dismay, neither she nor her mother have a penis. She then turns to the father in hopes of bearing a child by him that would substitute for her missing penis; the girl would become a mother in her mother's place. Thus, whereas castration ends the Oedipus complex for the boy, it begins it for the girl.
The Oedipal drama in its many permutations determines the course of the trilogy. Lavinia, for example, yearns to replace Christine as wife to her father and mother to her brother. Christine clings to Orin as that the "flesh and blood," entirely her own, that would make good on her castration. Brant, in turn, is but a substitute for her precious son. Orin yearns to re- establish his incestuous bond with his mother. But the war, where he would finally assume the Mannon name, forces him from their pre-Oedipal embrace in the first place.
Though titled after Electra, the predominant pair of lovers in Mourning is the Mother-Son. Put bluntly, the male Mannons in some way or another take their female love objects as Mother substitutes, and the women pose them as their sons. The Fathers of the play, Ezra and otherwise, figure as the rival who would break this bond of love. As we will see, what is primarily being mourned here is the loss of this love relation, this "lost island" where Mother and Son can be together.
As Travis Bogard notes, O'Neill wrote Mourning to convince modern audiences of the persistence of Fate. Accordingly, throughout the trilogy, the players will remark upon a strange agency driving them into their illicit love affairs, murders, and betrayals. What O'Neill terms fate is the repetition of a mythic structure of desire across the generations, the Oedipal drama.