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To clear up the mess you've caused --

the hierarchy are NOT the OUM, they may or may not EXERCISE OUM

Generally, the hierarchy exercise OM (without the U).  When the hierarchy agree on teaching a "matter of faith" in a constant, universal, and undisputed way IN UNION WITH THE POPE ... then they exercise OUM, which is infallible.

Simply because a man is in office doesn't mean that he chooses to exercise ANY teaching authority.  We've had popes who taught absolutely nothing but preferred instead to give themselves to their own worldly pursuits (hunting or chasing mistresses).

So the men in office are merely ORGANS of the OM, in potency.  It's only when they actually teach that they exercise Magisterium.

You keep confounding the office holders with their active use of teaching authority.  This teaching authority exists in potency only, and may or may not even be activated.

But as to what constitutes an infallible definition or teaching of the OUM, that can be a little murky sometimes --

Catholic Encyclopedia:

And while for subsequent ages down to our own day it continues to be theoretically true that the Churchmay, by the exercise of this ordinary teaching authority arrive at a final and infallible decision regarding doctrinal questions, it is true at the same time that in practice it may be impossible to prove conclusively that such unanimity as may exist has a strictly definitive value in any particular case, unless it has been embodied in a decree of an ecuмenical council, or in the ex cathedra teaching of the pope, or, at least, in some definite formula such as the Athanasian Creed. Hence, for practical purposes and in so far as the special question of infallibility is concerned, we may neglect the so called magisterium ordinarium ("ordinary magisterium") and confine our attention to ecuмenical councils and the pope.

Similarly, when there's no Pope, such as in sedevacantist periods, the OUM continues to exist IN POTENCY, and that's why the Magisterium does not defect simply because there's no Pope who happens to be formally exercising it.  It's no different than if there WERE a Pope in office who chose not to teach anything.

So you're confounding authority in potency with authority in act.

Teaching and Governing are merely aspects of the same Office.

So what?  The teaching office is related to the spiritual/doctrinal/Divine/unchanging nature of the papacy, while the Governing is related to the human/fallible/can-be-changed portion of it (i.e. Christ gave Peter the power to "bind and loose").

The OUM is infallible and part of the Teaching office of the papacy.  The OM can either be infallible or fallible, depending on how's it's used.  The OM is infallible when it reiterates what has "always been taught" as +JPII did when he reiterated the bad on women priests.  The OM is fallible when it is not clear that what is being taught is Traditional, Apostolic and "universally believed" as many of the encyclicals of the post-V2 decades have been, since they are filled with a mix of orthodoxy and novelty and thus their ambiguity undermine their authority.

The OUM necessarily is obliged to be believed with certainty of faith, since it is a doctrinal, Apostolic teaching of the highest degree, infallibly protected.  The OM has varying, multiple levels of authority attached to it, depending on how the pope teaches, what he references and if he makes use of Apostolic teaching authority, or if he just teaches as a normal Bishop and private theologian.  Plus, since the 50s, the definitions and labels for what the magisterium actually is, have changed, been debated and further examined.  To date, the Church has not clearly explained the various levels of the magisterium.

Suffice it to say, the Church has an obligation to make it VERY CLEAR that teaching x, y or z is obligatory and to what degree.  If they do not, we are allowed to ask for clarification because Scripture says that "there is nothing new under the sun" and apparent novelties have a right to be challenged until the pope teaches with authority.  Those heretics who ran V2 used the faithful's trusting nature and docility against them, to imply that V2 was doctrinal, when we now know (from many statements from those in attendance) that it was anything but, and its heretical novelties should be done away with and anathematized...which a future orthodox pope will do someday.

Notice the way Msgr. Noort says, "the ordinary and universal magisterium is giving an utterly clear-cut witness to the legitimacy of his succession." - so Teaching Body, Hierarchy, OUM can be used synonymously.

But let's rephrase the question, then: where are the Ordinaries of the Church? Only those who are Ordinaries (1) can exercise Ordinary Jurisdiction by virtue of their Ruling Office/Ruling Authority (2) can exercise Ordinary Teaching Authority by virtue of that same Office.

So what?  The teaching office is related to the spiritual/doctrinal/Divine/unchanging nature of the papacy, while the Governing is related to the human/fallible/can-be-changed portion of it (i.e. Christ gave Peter the power to "bind and loose").

Not so. Read the citation from the CE again. When Vatican I defined the supreme teaching authority of the Roman Pontiff, it also defined his universal jurisdiction. The two are related, as the CE puts it, "Since, however, the teaching of the Church is authoritative, the teaching authority is traditionally included in the ruling authority". They are only One Office, its governmental aspects and its teaching aspects are inter-connected.

Similarly, when there's no Pope, such as in sedevacantist periods, the OUM continues to exist IN POTENCY

The Bishops retain the powers they have already received from the Pope before he died. "Then indeed the divine law and institution of perpetuity remains, and by the same reason the right and duty in the Church of procuring the succession according to the established law; there remain also the participations in the powers [of the papacy] to the extent they are communicable to others [e.g. to the Cardinals or bishops], and have been communicated by the successor of Peter while still alive, or have been lawfully established and not abrogated [thus the jurisdiction of bishops, granted by the Pope, does not cease when he dies]; but the highest power itself, together with its rights and prerogatives, which can in no way exist except in the one individual heir of Peter, now actually belong to no one while the See is vacant." (Cardinal Franzelin, VACANCY OF THE APOSTOLIC SEE, no. 15) How will you explain what Van Noort said?

You assume that the Novus Ordo hierarchy exercise the OUM

The Catholic Hierarchy is identified by the Bishops appointed to office by the Pope. Where are they, otherwise? There's no Hierarchy either?

We are bound no less by either the Church's Ordinary Magisterium, Universal Magisterium, or her Extraordinary Magisterium. No less because what we're bound by is Catholic truth, it does not matter the method, it is the matter, the truth that binds us.

You keep citing Lumen Gentium 25, but you state something not stated there. When something is taught by the Extraordinary Magisterium (e.g. Trent), we owe it the assent of divine and Catholic Faith. When something is taught by the Ordinary and Universal Magisterium (e.g. all the Bishops stating the Assumption of Mother Mary is divinely revealed dogma), we again owe it the assent of divine and Catholic Faith. This is clearly taught in Vatican I, and is cited by Pope Pius XII to that effect. When something is taught by the Ordinary Non-Infallible Authentic Magisterium, we give it a prudential and conditional assent called religious submission. https://en.wikipedia.org/wiki/Obsequium_religiosum

Since the magisterium really is always infallible

Magisterial statements are not always infallible, but they are generally safe. Not everything becomes infallible just because it occurs once in a Papal Encyclical. There are many theological grades of certitude like doctrina catholica, proxima fidei etc before a doctrine becomes de fide. You are attributing to me something you said above, "We are bound no less by either the Church's Ordinary Magisterium, Universal Magisterium, or her Extraordinary Magisterium." I don't think that's right. Assent of divine and Catholic Faith we give to an ex cathedra and infallible teaching, and Religious Submission we give to all teaching that is not definitive or infallible.