"O Jesus, Living in Mary" is the beginning of a prayer composed by Fr. Jean-Jacques Olier, the founder of the seminary of Saint-Sulpice in Paris (where St. Louis Marie studied for the priesthood), and a leading member of the "French School of Spirituality". But the phrase "Living in Mary" also sums up the interior attitude of one who has mastered the art of living to the full the total consecration of oneself to Jesus through Mary. It is the putting into practice of the fourth of St. Louis Marie's "interior practices" of true devotion to Mary, doing all that we do in Mary.
According to St. Louis Marie, this means recognising Mary as the "dwelling place of God", the "paradise" of God, since she received him in her womb and (more importantly) in her heart, and trying to enter into the heart of Mary, resting there, relying on her, hiding within her and abandoning ourselves to her care. It can be appreciated that this is something of a "mystical" notion, and it is not everyone who will be able to achieve this. In fact, St. Louis Marie stresses that it is only by a special grace of the Holy Spirit, that we can enter thus into the heart of Mary:
The unfortunate children of Adam and Eve driven from the earthly paradise, can enter this new paradise only by a special grace of the Holy Spirit which they have to merit (TD 263).
We should, however, if we are serious about living our total consecration to Jesus through Mary, try to attain this sense of "living in Mary". It is a question of an intimate union between Mary and ourselves, so that, in whatever we do, we try to be as united with Mary's motives and dispositions as it is possible to be.
An interesting analogy which may help us to understand this idea is in the DVD, "Jesus Living in Mary: The Spirituality of St Louis Marie de Montfort." You can watch this section of the video below: