My preliminary conclusion regarding Oerth and the Planes is this: Oerth exists in a
sort of planar cul-de-sac which has limited connections (we actually only know of one) to
the rest of the multiverse. I have only been able to travel to five planes from Oerth
and from none of these planes (or from Oerth itself) have I been able to create gates to
any of the planes I am familiar with.
The five planes in question are the Elemental Planes of Earth, Air, Fire and Water and
a 'Celestial' plane which appears to be an all purpose Ethereal/Astral/Shadow/Celestial/Infernal
Plane which has some interesting properties which I will describe below. The nature of this
plane and Oerth's relationship to it leads me to suspect that Thatis's speculation that
the Oerth has its own Gods may well be correct.
The first area of interest in the 'Celestial' plane is what the scant Oerth literature on
the subject calls the Shadowlands. This area has many of the characteristics of the
Ethereal and Shadow planes of the main multiverse. It appears to be an area where
the dead gather before they move on to other realms. The geography of this area is deceptive
in that it appears to be a copy of Oerth (as you would expect from an Ethereal Plane) but
in fact it is much larger. If you draw a straight line in the dirt there are at least three
different directions one can go at right angles to that line (excluding up and down). Also if
you turn round 360 degrees on a single spot more than once you do not always see the same view.
The 'surplus' direction will take you away from the copy of Earth towards what I guess
are the areas in which the Gods dwell and which I assume have been created by divine power
morphing the celestial plane to reflect the desires of the individual Gods. So in this
respect the Plane is also an Astral Plane in that is connects areas which have many of
the characteristics of the known planes in the multiverse. The difference is that one walks
through a landscape to get anywhere.
The interesting question all this raises is that if the above is an accurate description of
Oerth's place in the multiverse why does it have such strong cultural connections to
the main multiverse? A speculation might be that in the distant past Oerth had better
planar connections or that there were more gates (perhaps divinely created) which have been
now lost. The Gods of Oerth (if indeed they are beings independent of their counterparts in the
rest of the multiverse) may have shaped themselves to reflect the cultural memories of
the races migrating to Oerth. Clearly there is a lot more work to do to understand Oerth's
history!