These rules mention of the Planes of Existence in many places, but for the purposes of the OSRIC core rules, the authors do not intend to explain very much about them. This is deliberate—the planes are intentionally left blank as a possible route for future creativity. They are for higher-level play (for characters of at least 10th level), when play in the normal game world should be growing too easy. The OSRIC core rules game balance begins to break down at higher levels than this, though enjoyable adventuring in carefully-designed environments may still be possible.
At this stage, suffice it to say that:
The normal campaign world is situated on the Prime Material Plane, wherein things and creatures are generally made of matter (hence "material"). There are two planes immediately contiguous with the Prime Material Plane: the astral and æthereal planes.
These planes are misty, vaporous places. It is possible dimly to perceive the Prime Material Plane from the astral or the æthereal, but only as shadowy and indistinct shapes and forms. A creature the size of a human is only visible within about 30-ft of the viewer. While certain magical creatures can perceive the astral or æthereal planes or even attack within them, apart from this a character on an alternative plane is completely imperceptible: not just invisible, but silent and similarly concealed from all senses.
A character can use the astral or æthereal planes to pass through solids such as walls on the Prime Material. Such things are not solid at all on these planes. If the character is "inside" a solid object via this route, visibility is zero; the character is effectively blind. He or she must emerge into some open area, or return to the Prime Material will be fatal, no saving throw.
When a player character enters a new plane, he or she makes a "bloink", like the splash when someone jumps into water. Powerful hostile creatures on the planes can detect the "bloink" and will move to intercept. A character below 10th level may remain on another plane for up to 1d6 turns. Beyond that, assume he or she is consumed by the astral/æthereal equivalent of a grue. In other words, that character is gone, permanently and irrevocably destroyed without any possibility of raising or resurrection short of a wish.
Certain other planes (the elemental planes, the abyss, the hells, the negative material plane, and so on) are mentioned from time to time. These references are deliberately left obscure.
In the planes, things are different. The core rules will not necessarily apply. Magic items and spells may not function as they would elsewhere. "To hit" and damage rolls may vary, as may class abilities, saving throws or indeed anything else at all.
When a character above 10th level desires to explore the planes, as will eventually happen in a long-running campaign, the GM will need to determine what happens there.
By that stage, the authors hope, the GM will be sufficiently experienced to cope with the situation and indeed enjoy rising to the challenge.
The authors would like, in the fullness of time, to release an optional supplement or supplements about the planes, but this will not form part of the OSRIC core rules.