Astral Projection for OSR games

Disclaimer: various editions of D&D have their own vision of the astral plane. I don't much care about them, this is a different thing.
Disclaimer 2: This is statted up for LotFP.

How do you get to the astral plane?
You need to achieve an altered state of conciousness so that your mind can leave your body and travel to the realm of pure concepts. There are four notable ways of doing this:

You can take Exotic Psychedelic Drugs that let you slip into a drug-fuelled cosmic trip, entering the Astral Plane. You'll probably need to spend around 500 silver (or gold in a gold standard) to find the good stuff and be able to trip, but actually taking the drugs can be done in around a minute.

If your body is brought to - but not beyond - the point of death and then preserved in that state (perhaps through time magic, freezing cryogenic style, or similar) the soul can slip free of the body until the body is brought back to life. This is, needless to say, quite dangerous unless you've got a skilled doctor/vivimancer on hand.

If you're a member of a class like the Monk that gets powers flavoured around chi, zen, meditation and other such eastern martial artsy stuff: if you have the knowledge (IE a teacher or past experience projecting through another means), you can project simply by meditating for a turn.

Spells or magic items probably exist that do the job too.

What Is The Astral Plane Like?
The astral plane is composed of ideas, not things. You can see the material world dimly, but not as it appears to material beings. Rather, you see symbols of whatever is on the material plane hanging in astral space. Things are platonic ideals, not actual objects Things shift and distort as your perception of them changes.

An example. You see a house in the distance: it is a simple symbolic representation of a house: triangular roof, square windows, door, chimney. As you approach, the symbol-house splits into four walls (flat 2d brick surfaces) hanging in space, with the images of doors and windows intersecting with them. Step into the house and now the various contents 'pop in': a chair floating in the mid-room, gently orbiting a table.

What Is Your Astral Self Like?

When you first appear on the astral plane, your projected conciousness takes a form resembling your self-image. This will likely be based on your normal physical appearance. However, more important than how you actually look is how you - deep down - conceptualise yourself. An elderly character likely appears how they did in their youth, for example. 

Your body normally appears naked and without equipment. Clerics (and other characters that use divine magic such as paladins) appear with the holy symbol of their god, however.

A spellcaster appears with the spells they have memorised. Each takes the form of a flickering image of the spell's potential (a little flame for a Fireball, for example) that rapidly orbits the spellcaster. Each memorised spell appears in an orbit-shell corresponding to its spell level: a spellcaster has distinct 'layers' of spells orbiting them much like the way electrons orbiting an atom fall into distinct energy levels.

Each such astral form is linked back to its physical form by a thing silver thread that extends from the back of the head to the body's physical location.

You can, in time, learn to change what your astral form looks like. Your silver cord always remains, however.

Mind-Sense

Every astral projector perceives the thoughts of all other astral projectors present. They feel - one step removed - their emotions and hear their inner monologue. What this means in practical terms is that your PCs hear the table-chatter of the players. Obviously, these things don't translate directly. Firstly, references to game mechanics that abstract things in the world-fiction are percieved in the terms the PCs would understand it. (Example: 'I'm a third-level fighter' translates to 'I'm a knight of some renown' and 'I keep rolling badly' translates to 'I'm not feeling on top form today'.). Secondly, references to things outside the game (IE who's turn it is to buy pizza, social chatter, etc) are not perceived. Outside of this, however, you can assume that if a player says something, that thought is in their PC's mind, and thus the other PCs become aware of it. (If your PC has secrets to keep, then you should keep your mouth shut OOC or else the other players will become aware of the surfacing memories).

To simulate the way that PCs can sense the minds of astrally-projecting NPCs nearby, the GM should be describing out loud their decision making process for what NPCs do.

Your Stats on the Astral Plane

Rather than your normal stats, use the following:

• You have a single Astral Hit Point per level, regardless of class etc. 
• You completely ignore your physical stats: Strength, Dexterity and Constitution do absolutely nothing.
• You cannot cast spells. Instead, each memorized spell does three things.
• You get +1 Astral Hit Point per spell memorized.
• You can spend a memorized spell to get its spell level as a bonus to any roll you make.
• Your memorized spells act as a beacon to astral predators. Anything attacking you can likewise spend your memorized spells for the same bonus. They feed off the mental energy coded in the spells.
• Things like thief skills are mostly useless, unless you get very weird and metaphorical with them.
• Your AC is depends on what sort of attack is being used against you. 
• Those that attack your base assumptions about the external world use your intelligence score as your AC.
• Those that attack your ability to accurately perceive the world use your wisdom score as your AC.
• Those that attack your sense of self use your charisma score as your AC.
• Your attack bonus depends on the type of attack you're using.
• If you're attacking your victim's assumptions about the external world, use your intelligence bonus for the to-hit roll.
• If you're attacking their ability to accurately percieve the world around them, use your wisdom bonus for the to-hit roll.
• If you're attacking their sense of self, use your charisma bonus for the to-hit roll.
• Your silver cord has an AC equal to the best of your intelligence, wisdom or charisma.
• You have no gear. Or whatever your astral form has, but its just part of you, really, and really just a metaphor.

Astral Shapeshifting

You can change your astral form through a simple effort of will, re-conceptualising yourself so that your astral form shifts to match this new concept. 

Astral Shape-shifting in this way requires a concerted effort: roll under Charisma to succeed. If you fail, you lose one Astral HP. 

You can shift to a self-image you've taken before and accepted without a roll.

You can maintain as many self-images simultaneously as your Intelligence score. Once you've taken more than this many, you must discard and forget shapes to make room for new ones in your repertoire. 

If you modify an existing shape, that counts as an entirely new shape even if the differences are only slight.

(Example: Volg the Orc has an int of 5. She has taken the shapes of: her mortal form, a might wolf, that mighty wolf but on fire, a huge oak tree, and a rusted, blood-stained battle-axe. She can take any of these forms without needing to roll. If she then successfully shifts into the form of an orc child, she must forget how to take one of her other five shapes if she wants to be able to add the child-shape to her repertoire.)

However you shapeshift, your silver thread and the shells of orbiting spells always remain. Astral visitors can always be distinguished from natives by their thread, and individual spellcasters can often be identified by the cloud of spells that orbit them.

Astral Attacks

You attack other astral entities by thinking really, really hard at them, projecting an idea at them so intensely that they will accept it and incorporate it into their own thoughts: projecting harmful thoughts lets you attack the Astral HP of your victims. The nature of the psychic attack determines which stats are used as AC and the to-hit bonus. 

If your astral form is symbolically or conceptually able to 'beat' the form of your victim, you roll with advantage; roll twice and take the better number. (Example: if your astral form is made of fire, you get advantage against astral forms made of wood). The inverse is also true, if your astral form is weak to that of your victim, you roll with disadvantage; roll twice and take the worse result. (Example: if your form is made of ice and your victim's from fire).

Normally, an astral attack only causes one Astral HP to be lost. The GM will rule that two or even (in exceptional circumstances) three Astral HP are lost if you target the victim's specific psychological weaknesses. (Example: If your victim is arachnophobic, you do two damage if you make them believe that there are spiders under their skin).

You can attack a victim's silver cord rather than their normal psyche. Only attacks that target their belief that they can return to the physical world will work here. When the cord breaks, the victim can no longer return to their body without outside intervention to restore 'part of a specific physical being' to their concept of self. However, so long as you're happy to remain in the astral world, lack of a silver cord is no hindrance.

If the silver cord is broken, the body becomes empty. Other entities (that lack a silve cord of their own) can take up residence in it. 

Astral Actions

Each round you can do all of the following:

• Move to a specific place that you can conceptualise or are familiar with. No range limit. Observers see where you're going, and follow.
• Shift your astral form to a new one.
• Make a single astral attack.
• Think as much as you like.

Astral Inhabitants

The following can all be found on the astral plane.

Elementals, as conceptual entities, exist on the astral plane and physical plane simultaneously.

Many incorporeal undead have their true forms on the astral plane and project into the physical plane.

Strange beings are native to the astral plane. Use the stats of normal monsters to represent self-sentient concepts. 

Mortals can travel to the astral plane. Some are stuck there.

All such beings use the same rules that PCs do: One astral HP per hit dice, etc. Treat spell-like abilities as first-level memorized spells that refresh themselves at an appropriate rate. Since most monsters don't have listed Int/Wis/Cha scores, you'll need to assign scores. Put a 14 in the one they're best at, a 7 in the one they're worst at, and a 10 in the middle-score.