I am playing D&D 5th edition. While reading the DMG, on page 44, it says:
The Inner Planes form a wheel around the Material Plane, enveloped in the Ethereal Plane.
Using the Great Wheel Cosmology, does this mean that the Ethereal Plane does not exist around the Outer Realms?
Could one cast etherealness or use "Oil of Etherealness" in the Outer Planes?
I would also be curious if other spells would be affected, but in the interest of specificity, I am just asking about etherealness.
The Etherealness spell states, in part:
This spell has no effect if you cast it while you are on the Ethereal Plane or a plane that doesn't border it, such as one of the Outer Planes.
So no, the Outer Planes don't border the Ethereal, and furthermore the Etherealness spell has no effect when used there.
No.
If you look at page 48-49 in the DMG, you will see that the Ethereal Plane borders only the Inner Planes, Material Plane, Shadowfell and Feywild.
This was also the case in some of the earlier editions of D&D that featured the Great Wheel Cosmology. For example in the 2e Planescape Campaign Setting explicitly stated that the Outer Planes are cut off from the Ethereal. Similarly, the 3.5e Planar Handbook described the Astral Plane as a conduit to all other planes, while the Ethereal was mentioned only as means of transportation within the Material Plane it is coexistent with. The diagram on page 137 of that book shows that the Ethereal is not in contact with the Outer Planes.
Finally, regarding the Oil of Etherealness in particular: Page 46 of the DMG states that the spell Etherealness allows adventurers to enter the Ethereal Plane. Page 184 has a description of the oil, where we learn that the affected creature gains the effect of the etherealness spell. So, we can conclude that the oil is not going to function in the Outer Planes.
None of the Outer Planes are connected to the Ethereal Plane. Instead, they are connected via the Astral Plane.
The Astral Plane connects the Outer Planes, while the Ethereal Plane connects the Inner Planes. There is no connection between Astral and Ethereal. Maybe it would help your intuition of you think of this as the gap between "heaven" and "earth."
Right from your quote, it says this explicitly:
The Inner Planes form a wheel around the Material Plane, enveloped in the Ethereal Plane.
It also says on DMG 48:
The Ethereal Plane is a misty, fog-bound dimension. Its "shores," called the Border Ethereal, overlap the Material Plane and the Inner Planes (emphasis mine)
If you consult DMG 49, you will see that the Ethereal Curtains directly connect all of the Inner Planes, including the Elemental Chaos, with the Deep Ethereal. The Deep Ethereal is not its own plane, but rather the "deep" portion of the Ethereal Plane.
Finally, if you reference the image on PHB 303, you can see the Ethereal Plane connects with all of the Inner Planes.
Since the Elemental Chaos separates the Astral and Ethereal Planes, spells and abilities similar to Etherealness wouldn't have any effect while in the Outer Planes. - PHB, pg 303
You could use Etherealness while in the Elemental Chaos or any of the Elemental Planes however, since they are considered to be adjacent to the Ethereal Plane, just on the other side of it compared to the Inner Planes. You would be in a region of the Border Ethereal closer to the Elemental though and so you may have to worry about adverse conditions while there (consult your GM for guidance on this.)
If you find yourself in the Outer Planes and in need of an effect similar to Etherealness, you should consider using Astral Projection instead. This could potentially allow your character to slip into the Astral Plane in a similar way to how someone would slip into the Ethereal Plane from the Inner Planes, although with certain limitations, such as leaving your physical form behind, rather than transferring it with you. - PHB pg. 215
You could consult your GM about possible items such as the "Oil of Astralness", which in theory would act in a similar fashion to the "Oil of Etherealness", but such an item would likely be considered a homebrew by the GM, despite its relatively minor alteration in the functionality of the original item. It may be that exposing your mortal form to the raw power of the Astral Plane might have potentially harmful side effects, or in some way limit the functionality of the item being used.
Or it may transform your character into a deity.
