(By the way, this article was written last, while I was in a somewhat tongue-in-cheek mood.)
The Master of Spirit is vastly powerful. He can create whole realms and transform a spirit into anything he pleases. What can there possibly be that an Oracle of Spirit can do that a Master cannot?
Phenomenal Cosmic Powers and How to Get Their Attention
In essence, two things: First, the Master of Spirit may stand up to the Umbrood Lords, but even he is at the mercy of Incarna and Celestines. "'I can call spirits from the vasty deep.' 'Aye, so can I, and so can any man, but will they come when you do call them?'" The Master of Spirit may invoke such cosmic powers; he may call them (and perhaps they will give him their ear if he amuses them); but in truth he cannot compel them to come.
Not so the Oracle. Just as the Master has worked his way up the cosmic chain of being, gaining authority over the lesser powers of the Tellurian, so the Oracle walks among Powers unimaginable to the Sleepers and only dimly imaginable even to an ordinary mage. In some sense, he has even become one of them.
Not-So-Itty-Bitty Living Spaces and What to Do with Them
Second, while the Master of Spirit can create realms of his own, he is at a loss when it comes to a realm already established by someone else. He may build on its rules, perhaps, but he cannot tear them down or even alter them.
The Oracle has passed beyond this limitation as well. He has the power to alter, reformat, or demolish other realms entirely. He may suffer Paradox in his efforts to do so, but once the job is done he can work his will without the slightest fear that reality will strike back at him--because it is a reality entirely of his own making.
Rulership and Authority
These abilities are not so far apart as they may seem at first. While some realms, such as Horizon Realms, are the work of human mages, the majority are the creations of powerful spirits. Occasionally a Lord or even a Preceptor will set up shop in some relatively undefined corner of the Umbra; still, the greatest realms are the work of Incarna or even Celestines, created at their will to serve their whims.
Just as Resonance connects human mages to the realms they build, so it connects spirits as well, perhaps even more strongly in that many realms are the work of a single powerful being, whereas humans usually must work together. Some have even theorized that the present forms of the greater spirits are not original, but are the contamination of centuries or millenia occupying the realms they created. In any case, the result is that power over a spirit gives one power over its realm, if any, and vice versa.
Ecology of Spirit Realms
Moreover, realms are not generally occupied merely by a single being, nor are they merely empty rooms. They possess a spirit ecology all their own. These spiritual communities are dependent on their realm in much the same way as physical beings are dependent on theirs, and a mage who can shape the realm itself can shape its spiritual inhabitants to the same degree.
The beginning Archmage of Spirit relearns, in a sense, some of the things he learned as a mere Disciple--not only how to call a spirit, but how to treat a being more powerful than she is! The Archmage has passed beyond the realms of mortal magick, only to find that she is now a small fish in a big pond once more. Nonetheless, she is as capable of attracting the attention of an Incarna-level spirit as she once was of calling lesser beings. Generally, of course, the Incarna will not send its entire being, but only an Avatar of itself; nonetheless, its attention is at least partially occupied by the summons and it can bring its full power to bear on the situation if the mage so requests.
At this level, the Archmage also learns something new regarding the spirits she once worked with--they can be fused together into greater beings. This is similar to binding them together, but more profound; at the very least, the spirits are becoming a group entity, though they can be so fully joined as to lose all individuality. At this level, the Archmage is only capable of performing this on Minions of a greater being (who may not appreciate it), or to use the Garou terms, Gafflings and Jagglings. However, she is capable of joining them to their patron, generally a Preceptor or Lord. The resulting being in a merger can itself reach the level of a Preceptor or Lord, which may impair the mage's ability to deal with it somewhat.
Finally, the Archmage begins to learn how the structure of a realm someone has already created can be altered. At this point, only Deep Umbral realms can be changed, and only to a small degree; the Archmage can alter the paradigm of the realm to suit herself, but its basic relation to Paradox remains unchanged, just as it was created. Take note that the Near Umbra is itself a Deep Umbral realm, and can be altered as such. (As one result, the Archmage is now able to set up full realms on the order of Horizon Realms within any part of the Near Umbra, not mere Domains.)
Having learned to summon an Incarna, the Archmage is now faced with the task of learning to control it. Just as with the lesser spirits, Incarna can be awake or in Slumber; most known Incarna are awake, but it may be suspected that realms that do not appear to possess rulers hide sleeping Incarna (or, in the farther realms, Celestines). The Archmage has now gained sufficient influence in the spirit world that he can put an Incarna to sleep, or rouse one if he finds it already Slumbering. Likewise, the mage learns to bind an Incarna just as he would bind any other spirit. Incarna rarely take well to being bound, and they retain a greater degree of awareness while bound than even an Umbrood Lord would, so even these powerful mages take care in their dealings with such beings. Moreover, any Avatars the Incarna might have separated from itself at the time are treated as separate beings on the level of Preceptors or Lords, except that they know what has happened.... Just as a fetish containing an ordinary spirit confers one or two of that spirit's gifts on the object, so a fetish containing an Incarna confers a small amount of its power, usually a low level of one or two Spheres. Being ridden by an Incarna (and only the most foolhardy Archmagi attempt it) confers all of its powers, just as with a lesser spirit, and greater physical advantages than in such a case, to allow some small degree of safety for the ridden person.
Also, as the Archmage previously learned to fuse Minions into a single being, so can he now merge Preceptors and Lords. Essentially the same process is occurring, with the resulting being no more powerful than an Incarna. Of course, just as the Archmage could previously fuse a being's Minions into it, so he can now join an Incarna's "minions" to its being.
The Oracle's power over Incarna continues to increase as she better understands them. At this level, she becomes capable of changing an Incarna's form and nature just as a Master can do with lesser beings. This magick is not at all undertaken lightly; remember that an Incarna represents a powerful concept. Transforming such a being almost invariably has repercussions on that concept, and thereby on the rest of reality.
Moreover, the Oracle can now summon the Celestines themselves. Like Incarna, the Celestines will not manifest their entire being before the mage, but she will certainly have their attention. Naturally, summoning a Celestine is incredibly dangerous, even for someone as powerful as an Oracle. If the mage has gotten cocky about dealing with spirits (and it does happen; consider what she can do to most of them by this point), she likely will not survive her first encounter at this level.
As her power over individual spirits grows, so does her power over the realms they dwell in. The Oracle has now mastered her control over Deep Umbral realms; she can not only alter their paradigm to change what is coincidental and what is vulgar, she can actually remove Paradox from them entirely. If this sounds too good to be true, it may well be so--the simplest way to eradicate Paradox is to wipe the realm clean like the Deep Umbra, essentially destroying the portion so changed. Though little is known about the Oracles' activities, there is some evidence that even more gentle methods of removing Paradox completely may result in damage to the realm over the long run; reality has an immune system for a reason. But the mage is also capable of establishing her own boundaries of Paradox, which presumably is enough to keep the realm going--essentially, she can now create Deep Umbral realms inside a Near Umbra.
The Oracle has also learned how to deal with the system of reality deeper in. She now has the ability to tamper with the paradigms and spirit ecologies of Near Umbral realms just as she could with Deep Umbral ones previously. The maximum extent of such an alteration would be to make all magick coincidental, just like in Earth's Near Umbra. Yes, the Earth does qualify as a Near Umbral realm, and the Oracle who ventures there can alter the local paradigm at will and control the _spiritual_ (not physical) ecology of the area. Fortunately, the whole planet is just too big for even an Oracle to change the whole thing without a lot of time and effort. Moreover, tampering with the paradigm is itself vulgar in nearly every reality imaginable, so the Oracles don't enjoy complete immunity.
As he learned to meld the lesser beings into one, the Oracle can now fuse Incarna as well. The consequences of doing that to such concept-beings are virtually unimaginable to mere mortals. Fused Incarna may be as powerful as Celestines in their own right. Also, just as before, the Oracle may fuse Incarna to their parent Celestine.
The Oracle also gains the ability to wake Celestines or put them to sleep. In theory, an Oracle of Spirit at this level is capable of waking up Gaia herself; that it hasn't happened is interesting, considering how many Spirit Oracles were presumably once Dreamspeakers. Moreover, he can bind Celestines as he can lesser beings. If allowing an Incarna to ride you is like proclaiming yourself a Nephandus before the Council of Nine, doing so with a Celestine is like prosecuting yourself. It does, however, allow access to the full range of the beings powers and confer the ability to (in theory) survive, and there are legends of Oracles who did it long ago. A fetish created from a Celestine would by no means confine it to that object, even less than with an Incarna, but it would bestow appropriate powers, generally one or two Spheres of levels up to Mastery, or Disciple levels of even all nine Spheres. Probably none have ever been created; the hubris of such an act would be immense.
The ultimate mastery of Spirit allows the Oracle to transform the Celestines themselves. Given that these beings are so powerful as to have been worshipped as gods, it should be clear that doing so would shatter reality in unimaginable ways.
Moreover, the Oracle can transform the paradigm of any realm, even Earth, to the extent of eradicating Paradox utterly. Again, doing that is unhealthy for the realm, but the Oracle is just as capable of establishing whatever rules she wants. (Doing so is still just as vulgar as ever; the result of a Paradox at this level is virtually incomprehensible. Imagine being taken note of by the Weaver herself...) It's also possible to go just a little further the other way, creating realms within which even magick, or certain forms of it, is impossible. Not vulgar; it just doesn't work.
Finally, the Oracle also gains the single most coveted power in the history of magick--the ability to Awaken any Avatar. No strings attached, although if the ex-Sleeper pays no attention, he may just go back to Sleep. Except for the possibility that the Awakening will derange the person involved, anyway...
Some Final Notes
Why not the ability to fuse Celestines? Well, what would they fuse into, except the One/the Source or some partial facsimile thereof? Further, as you may have noticed, none of these Spheres actually provides the absolute, total control over everything in their realm. If you want that, you'll have to go ahead and Ascend; some say that itself is the same as fusing with the One, after all.
Likewise, I'm not providing any data on travel beyond the Deep Umbra, for the simple reason that I can't imagine what could possibly be beyond the Deep Umbra. If you can, just divide up the levels accordingly.
In regard to fusing spirits, how does one go about judging the consequences of fusing the Avatars of several beings? Well...the quick and dirty answer is, don't do it. But since these supplements are being provided for a reason, I'm sure some crafty Storyteller will do it to someone eventually.
First--regard an Avatar as being on a level as follows: Disciple=Minion, Adept/Master=Preceptor, Oracle (6-8)=Lord, Oracle (9-10)=Incarna. The mage herself, as a whole being, is approximately a level higher, but that's relevant only to put matters in perspective.
Second, deal with Avatar Essence the same way you would deal with Nature and Demeanor in a group mind: fudge it. Just do what sounds right.
Third, if the problem should come up, consider a vampire's Generation (or Dharma), a shapeshifter's auspice and permanent Gnosis, a wraith's Eidolon, and a changeling's fae nature and permanent Glamour (I think there's such a trait; I'm not familiar with the Dreaming) as equivalents to a mage's Avatar for these purposes.