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Game Masters' GuideOSR+ Philosophy

Design Principles

Below is a summary of the design principles we followed when designing OSR+.

If you embark upon homebrewing the game, these principles will help you to avoid introducing mechanics that violate the integrity of the system.

Guiding Principles

Below are guiding principles that affect the rules in general.

Rulings Over Rules

Mechanics should be additive to the core rules and open to interpretation. The GM should be able to use the rules as tools to render rulings in unique situation, rather than as prescriptions for play.

Keep RUles modular

Kits, classes, and origins should not make assumptions about how a player might choose to build a hero. Players should be able to build character concepts from smaller building blocks (origins plus classes plus kits), rather than from fully featured "classes" or "playbooks" that dictate the concept in a single profile.

Keep Any Action Possible

The GM should be able to simulate any action a character takes with some combination of core rules, and barring supernatural actions, a player should be able to have his character take any action imaginable. The mechanics only ever enhance or hinder an action a character can take, not prescribe it. For example, having a skill enhances a skill check by adding a +2, but anyone can attempt a skill check even if they're not proficient in the skill. Similarly, making a called shot is entirely possible for a character without the Precise tactic, they just roll with disadvantage.


No mechanic should exist that adds modifiers to a roll, outside of the +2 that is granted by skills, and the situational advantage/disadvantage that may be granted by the GM via rulings. No spell, treasure, kit, or similar mechanic should exist purely to grant modifiers to checks.

Add New Choices, Don't Restrict Existing ones

Rules should be written succinctly and in plain language. Mechanics should evoke or suggest character concepts by providing new choices, rather than by outright defining them or by restricting existing choices. For example: a Tower Knight may have more options when using a shield than other heroes, but all heroes can still use shields.

Enable EASY gm PREP

It should be possible for a GM to design set pieces for encounters in minutes rather than hours. A shorthand for writing up the stat blocks should make it possible to describe an NPC in no more than a paragraph of text and bulleted lists.

Respect the limits of Players' active memory

Players shouldn't have to carry a lot of information in their heads at any given time; they should never have to divide or subtract to calculate a roll; and they shouldn't have to track minutia, such as a complex inventory with ammunition.

Avoid Setting-Based Lore

Mechanics in OSR+ (aside from origins in the core fantasy rules) should not reference setting-based lore, so that they can be used to support any genre of fantasy setting.

On Character Abilities

Character abilities include abilities from kits, techniques from classes, talents from origins, and skills, as well as the abilities granted by spells or treasures.

When designing OSR+, we didn't want to fall into the "feats trap" that many traditional games fall into.

Lean on the Core Rules

All abilities should be no more than a sentence or two long. Such a small paragraph should be written concisely, relying on the assumptions of the core mechanics, glossary keywords, or GM rulings to fill in the gaps. Remember, creative play happens when players take advantage of what is not defined explicitly by the rules.


No ability should replicate what a spell or treasure does without inventing on the premise or suggesting a character concept that isn't possible without first being a spellcaster. Spells always make the impossible possible, and they do so in very specific ways, so it's often duplicative (or reductive) for other abilities to do the same things they do. The reverse is not true, however: spells may replicate other abilities, provided the spell version of the ability is mechanically more costly to use, whether by increased MP cost or by imposing other limitations within the spell.

avoid Mechanizing Theater of Mind

No ability should exist that can be accomplished through theater of mind and core rules: abilities must break an existing rule to justify their existence, or make possible something that isn't within the core rules. For example: it's already possible to trip a rampaging goliath with an Hand-to-Hand attack as a called shot, so there's no need to introduce a "trip ability" into the system.


No mechanic should stand in for what can be done through a skill. For example, "picking a lock" can be resolved a number of ways with existing core mechanics (typically enhanced by the Reflexes skill), so there is no need for a separate ability that resolves "picking locks."


No mechanic should be so powerful that it can't be acquired at character creation. Because of this, there are no abilities with prerequisites in OSR+, although some abilities may scale with level. For example, every spell is available at any level, but spellcasters can increase or decrease the parameters of spells by investing more MP at casting time, which depends on their attributes increasing, which happens in turn when they level up.

Keep mechanics spam-proof

A mechanic that positively modifies a roll must always impose a limitation that prevents it from being used freely or continuously, for example, by adding an MP cost, setting specific conditions under which it may be used, or restricting the ability to only being usable a certain number of times per session or encounter.

encourage Players to Interrogate the Fiction

No mechanic should replace roleplaying. This is why there isn't a "reaction" roll in OSR+ or a "find / disarm traps" mechanic that a player can use to directly solve for the uncertainty in the fiction. Players only trigger mechanics when they interrogate the fiction manually, and then the GM selects an apporiate mechanic to resolve the uncertainty: they describe how they engage with the trap; they examine some specific part of the room to look for clues; they propose a specific argument to an NPC to convince them. Only then does a Perception check surface clues or activate the social combat mechanic.

Only Origins Should Grant Broad advantage & Disadvantage on Attributes

Only origins should give broad advantage or disadvantage to attributes, and each origin must provide one of both (or a similar global disadvantage to balance an attribute advantage).

ClassEs, Then Kits

Abilities from kits should be less powerful (more mechanically specific) than abilities from classes (which are more broadly applicable). Kits typically provide a single active ability and one or more passive abilities.

On Magic

Magic has its own logic in the core rules, but is also governed by the following design principles.

We had a number of goals when designing OSR+'s magic system:

  • First, we wanted players to be able to choose any spell at first level, because oftentimes tables never get to high enough levels to choose the really cool spells.
  • Second, we wanted to avoid introducing 200 versions of Fireball, so you'll find that no spell does damage in OSR+; instead, players can shape their maleficence to do harm, similar to how X-Men superheroes can control their powers to do creative things on the fly.
  • Finally, we wanted to keep magic items magical. There's nothing evocative about a +2 Sword. But a Dancing Sword that attacks of its own volition is certainly magical. Accordingly, you won't find any treasures in OSR+ that exist solely to impart mechanical benefits.

Treasure & Spells Create Options, Not Modifiers

Treasures and spells should provide heroes with new abilities, not modifiers to rolls. The point of magic is let players do impossible things (like fly or turn into ectoplasm!) so if all a spell does is modify a roll, it isn't all that magical.

Spells Don't Cause Harm

Causing harm with magic is the domain of maleficence, so no spell or treasure's ability should ever primarily deal damage.

no Instant Death

Nothing is more damaging to play than taking a player out of the action. Spells should never instantly kill players or NPCs. Similarly, any spell that incapacitates should always provide a chance to resist its effects each round, such that the player or NPC can get back into the action more easily.

no Fighter-mages

Spellcasters should never be as competent at combat as non-spellcasters, without significant trade offs. The same is true of non-spellcasters who try to use magic as competently as spellcasters.

Spell Logic

Generally, the wording of spells should rely on global spell logic in order to remain concise, and especially with respect to spell parameters, so that players and the GM can reason their way to sensible rulings without having to look up other core mechanics.

On Combat & Tactics

OSR+ tries to deliver quick, theater-of-mind combat while still preserving the turn-based, tactical flavor of traditional RPGs.

We didn't want GMs to have to look up situational rules to resolve whatever unpredictable maneuvers players might try pull off in combat. We also wanted to avoid making the action economy in combat encounters so complex that it would necessitate grids to keep track of what's happening on the battlefield.

Differentiate Weapons with Tactics

We differentiate weapon classes by balancing the amount of damage they do with the tactics they confer when using them situationally. This allows for combat maneuvers to arise in an emergent way during encounters.

Avoid Mechanizing Combat Maneuvers

A combat maneuver is, more or less, an attack that has been creatively re-imagined by the player. Any combat maneuver should be possible with the core rules alone, or some combination of the core rules and the use of a tactic. We didn't want to reduce combat maneuvers to mechanics because that would suggest that heroes would first need know the mechanic in order to perform the maneuver, which leads to button-pushing the character sheet on the part of the player.

Keep The Action Economy Simple

OSR+ only allows for a single action and a single movement to avoid complex attack sequences that can bog down encounters. A single action should never allow for more than two attacks in the round, except in rare cases where kits like the Marksman or Sellsword allow for a third, if the hero is also a Fighter.

Privilege Theater of Mind Play

The rules shouldn't require a grid to work out positioning, and relationships in space should be abstracted so as to avoid our having to use real-world measurements. Furthermore, we limit situational modifiers to advantage or disadvantage, to keep things like "cover" or "concealment" simple and predictable.

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