You are referring to different parts of the spiritual path. In the beginning it is vital to be able to discern the transient from the eternal. Most if not all are born identified with form and that seems natural for children. Form by nature is fleeting; so if we believe we are nothing but form (mainly body and mind), then we suffer because we are constantly under the notion that we are decaying and that which we are attached to is always trying to slip away. So devotion will be geared and directed towards self- preservation.
Therefore, it is important to ask a question that points to the awareness in spite of mind and body; in spite of form. If one devotes themselves to the inquiry relentlessly they will discover that they are the observer of phenomenon. And suffering starts to ease up; there are some good days and some bad days as the seeker is still living in duality and has not transcended the opposites. Nevertheless, there is some space now between the awareness and the transient thoughts and body sensations. A monumental feat in and of itself. This is the practice of mindfulness. There is an awareness that must be mindful of ego and its tomfoolery. What happens to the ego here is that it is exposed, examined, and its mechanics understood and put in its proper place.
Oneness speaks to a different part of the path in which awareness has merged into the ground from which all things spring. Here, a sense of an exclusive me (awareness is exclusive as well, although refined) is completely dissolved into the One Final Cause. This Final Cause is the animator of all that exists. This includes the sum of the egos. There is no “thing” outside of The Final Cause. Listen attentively now:
When the exclusive me merges with the Final Cause it becomes That. In other words, you become all things simultaneously but nothing exclusively. The simultaneous sum of all that exists equals the One. This includes the various activities of ego. From this simultaneous space, there is only spontaneous action that is born from the Final Cause itself. Since nothing is outside of It, there is sudden expression that is always congruent and in direct relationship with the moment. Ego still exists but it does not have an exclusive existence. It is only animated when natural harmony is dependent on it.
Here, diversity is not an obstacle to oneness. The actions of ego are made purified under the sole dictatorship of the One Simultaneous Final Cause. Which becomes you.