Old-School Essentials (OSE), by Necrotic Gnome, is a clean, meticulously laid-out edition of the 1981 Basic/Expert (B/X) rules — the same lineage that inspired so much of the modern OSR. It keeps the play style intact: quick to learn, lethal to characters, and rewarding to clever players.
The dice and the core mechanic
OSE uses the full polyhedral set, but the workhorses are the d20 (attacks and saves), the d6 (many dungeon procedures such as searching, listening and reaction rolls) and the d100 for thief skills and some tables. Six abilities — Strength, Intelligence, Wisdom, Dexterity, Constitution and Charisma — each give small modifiers, and armour is rated by Armour Class.
Because OSE reprints B/X honestly, it supports both the original THAC0 approach and a widely used ascending AC option. Both produce the same odds; they are just two ways of doing the same subtraction.
How a roll resolves
Attacks — roll high. Roll a d20, add modifiers, and try to hit the target’s Armour Class. In the ascending-AC style you add an attack bonus and aim to meet or beat the AC number, just like more modern d20 games. In the classic THAC0 style you compare your result on an attack matrix (THAC0 = “To Hit Armour Class 0”), subtracting the target’s AC to find the number you need. Either way, higher rolls hit better armour.
Ability checks — roll low. When the rules call for a raw ability test, the classic B/X method is to roll a d20 and try to get equal to or under the relevant ability score. A character with Strength 13 succeeds on a Strength check by rolling 13 or less. This roll-under-the-score logic is the opposite instinct to hitting a DC, and catches newcomers out.
Saving throws — by category. Rather than one save per ability, B/X characters have five named saving throw categories — against Death/Poison, Wands, Paralysis/Petrification, Breath attacks and Spells/Rods/Staves. Each has a target number on your class table that improves as you level; you roll a d20 and try to meet or beat that number.
| Roll | How you read it |
|---|---|
| Attack roll | d20 high — hit the target’s AC (ascending) or use the THAC0 matrix. |
| Ability check | d20 low — roll equal to or under the ability score. |
| Saving throw | d20 high — meet or beat the class table’s target for that category. |
| Dungeon actions | Often a d6 — searching, listening, forcing doors and reaction rolls. |
Note that these directions differ deliberately: attacks and saves want high, raw ability checks want low. It is worth stating out loud at the table which one is in play.
Effective strategy
Avoid fights you can win another way. Low-level characters have very few hit points, and a single blow can drop them. Experienced OSR players treat combat as a last resort — they scout, negotiate (the d6 reaction roll can turn monsters into allies or bystanders), and use the environment. The game rewards the clever, not the brave.
Play the dungeon, not the sheet. Much of old-school play happens off the dice: describing exactly how you probe a floor for traps, how you stack barrels, how you light the room. A good description can bypass a deadly roll entirely.
Manage resources ruthlessly. Torches, oil, rations, spell slots and hit points are all finite. Tracking light and time is not bookkeeping for its own sake — it is the tension the whole crawl runs on. Retreat with treasure beats dying for more of it.
Level through gold, not just kills. In B/X, recovered treasure is a major source of experience. That single design choice pushes smart parties towards getting the loot out rather than clearing every room.
Because every check is a single d20 (or a d6 for dungeon actions), you can roll a d20 in your browser and simply read it high or low as the moment demands.
Beginner tips & common mistakes
- Ability checks are roll-under. Don’t reflexively try to beat a DC on a raw ability test — you want to roll at or below the score.
- THAC0 and ascending AC are the same odds. Pick one convention for your table and stick to it; mixing them mid-session causes confusion.
- Saves are by category, not by ability. Learn your five save numbers; the right category matters when a spell or trap hits.
- First level is deadly. Play cautiously, keep spare character concepts handy, and don’t charge blindly into the dark.
- Use the reaction roll. Not every monster wants a fight — a d6 reaction can open a whole other path.
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