OSR games often feature rolling under attributes for pass/fail task resolving; it’s simpler than calculating bonuses from attributes, setting target numbers, etc.

Here is a mashup of OSR roll-under-attribute (specifically something like Whitehack) and PbtA pass/pass with consequences/fail with MC move.

When you take an action and the GM says you need to make a roll, it will be against one of your attributes. Roll a D20 and compare the result with the attribute number.

• if the result is higher than your attribute, you fail with consequences
• if the result is below your attribute and 10 or above, you succeed
• if the result is below your attribute but 9 or lower, you may succeed at cost
• if the result is exactly your attribute it’s a critical success

Cost or Consequences

Consequences happen when you try and fail (if there were no consequences, ask why you needed a die roll in the first place). Consequences can be made up by the GM on the spot or picked from a list (much like a MC move in Apocalypse World).

Success with a Cost is like paying Consequences to get the Success you wanted. The Cost of Success could be the same as the Consequences of Failure, or it could be different (usually less).

Difficulty

There are no numerical changes to the die rolls (either bonus or penalty). So how does the GM make the challenge easier or harder?

First, by changing the Costs of success for rolls below 10. If the Cost is a slider then setting it to zero means that a success with a roll below 10 is the same as a full success; alternatively if it’s set to “high” then it makes the chance of a Cost-free success lower, but also pushes a decision onto the player — take the hit now, or wait until the next opportunity to roll in the hope that you get a 10+ next time.

Second, by changing the Consequences. OK, this doesn’t affect the actual probability, but it does affect the perceived difficulty and pushes a decision onto the player. This only happens when the GM informs the player of potential Consequences in advance. It could even be phrased as “if you fail, XXX will happen” to set the stakes.

Third, by forcing Whitehack-style Disadvantage on the roll — so the player rolls 2 dice and keeps the lower result.

And fourth, by requiring more than one roll. You could demand a succession or rolls (for time passing and ticking bombs) or that all the rolls are made at once.

Skill and Expertise

That’s all well and good, but how does my character’s abilities affect this roll if there are no numerical modifiers?

The obvious one is rolling with advantage as used in Whitehack and D&D5e. You get to roll two D20s and keep the result you like.

The less obvious one is mitigating a Consequence or Cost. If you have a hierarchy of Costs, you could move the cost one rung down the ladder. Alternatively you could say the PC’s skill means they can defer one Cost per scene (or two, or more… though I’d stick with just one).

One thing this allows you to do is then ask the player how their PC is mitigating the cost — e.g. if they’re using an ability that lets them ignore this cost, where did this advantage or training come from? The approach should be (again) similar to Whitehack.

What about combat?

Since OSR has a whole subsystem devoted to fighting with AC, HP and BAB I guess you need to decide whether to keep this subsystem, or convert it.

If you convert it then you need to decide things like “does the GM roll dice, or just the players?” and how armour works, e.g. does it offset Cost or Consequence of a bad attack roll? I haven’t worked those out just yet, but I’ll get to them shortly.

Ladder of Costs

Finally, here are some PbtA style Costs aka MC moves:

• Take damage or trade damage (Cost can be mitigated by armour, hit dice, etc.)
• Put them on the spot
• Take their stuff
• GM advances a clock (or clock die)
• GM takes a pain token (Don’t Rest Your Head style)

Taking or trading damage can be according to a damage ladder, which is really just a way to differentiate between things that do some damage (e.g. a weapon in the hands of an average person) and more damage (a weapon used by a trained person, a bear, a dragon, etc.).

More generally some costs will be more onerous than others, hence the need for a “ladder” which will also allow the GM to tune the level of difficulty/consequence (q.v.). This is a WIP, so more later.