Red mold: dies from water, spore cloud if touched or burned, all within 20‘ risk 1d6x10 minute coma. Green mold: infects by touch, grows rapidly when wet, dies from burning.
In the stat-free writeup I used "risk" to signify a saving throw situation, which all editions but 4th have, and 4th can simulate with a Fortitude attack. Now, which saving throw? Really, I could leave the situation as is; the 3e system using Fortitude saves would obviously use those, and earlier editions can assimilate it to Poison. Nor would it do any harm to give some pointers in case the save to use is not obvious. For example, an arrow trap might be saved against using (Reflexes/Breath Weapon).
The green mold can give a similar (Fortitude/Poison) save when touched with bare flesh, maybe at -2 for the audacity to touch it. All editions' saves or save-like procedures, like hit rolls, use d20, making bonuses and penalties easy to apply. Like green slime, it will infect you and kill you in a certain amount of time ... let's say 1d6 hours ... provided it is not excised with fire (doing fire damage to the victim) or with a cure disease spell.
Okay, okay, "magical curing." But this brings me to the most vexing question. How to deal with spells, and magic items beyond the simple +2 sword? The very thickness of those sections in D&D rulebooks shows that there is no easy way to boil them down, though some have tried ...
Without going through my thought process in too much detail, here is my solution. Each OS document has a "base reference edition," or core rulebook it can refer to for itemized things such as spells, items and monsters, without having to stat them out.
Most of the time, I think, that will be enough; the DM using another reference can improvise or substitute from the name of the thing alone. If you want to deviate from your base reference edition, then give a short description of the new or outsourced spell's effects and other statistics.
A good default reference edition would be OD&D + Greyhawk (of which Swords and Wizardry is a reasonable facsimile). This is the source identified by Delta a while ago wherein most of the iconic spells in the game can be found. This spell list has shown remarkable persistence right through 3rd edition.
To sum up, Original Standard writes with these numerical and categorical mechanics:
Ability stats, 4 classes (race and class descriptors), hit dice or level, armor class (ascending and descending), damage (d8 longsword standard), move as a 12" human standard, saves either descriptive or (original/3e), coins on a gp base, time in rounds or normal units, modifiers to all the above stats, and a base reference edition that can be used to refer to itemized game elements like spells and monsters.
So tell me what I missed, and if this is something you can use - and if not, why not?