OSR Play Style 101
Introduction
Hello there. Several months back I wrote a blog post designed to introduce new people to the roleplaying game Cairn. To preface my specific guidance for that game, I wrote a brief primer for the OSR play style that Cairn is a part of. I found that a surprising number of people beyond those interested in Cairn ended up reading that blog post and sending it to others as a way to introduce them to the style of play.
Thus, here is a slightly altered version of that section of the blog post. May it lead to many dungeon delves and wilderness explorations.
What Does It Stand For?
OSR stands for Old School Revival or Old School Renaissance, depending on who you ask. There's a lot of argument over what the OSR is, how old its principles actually are, whether it is dead, etc. Regardless of its origins or exact definition, it is a very different play style compared to anything that people who started with 3rd edition D&D (me) or later are likely to have developed on their own. This is my attempt to briefly explain some of the core conceits and mechanics of the play style that often seem to trip people up. It is not an exhaustive list by any means, and any attempt to make it so would just be rewriting an existing OSR primer.
Conceit #1: Characters Earn Originality
In a lot of more traditional roleplaying games, you spend a lot of time creating an original character concept, writing up backstories of varying lengths, and using elaborate character creation systems that are designed to facilitate this originality by providing opportunities for customization. A character in an OSR game can typically be generated in a few dice rolls and some scrawled notes on an index card. This is important because the world doesn't care about your character in an OSR game. Bad decisions, or bad luck, can kill your character and derail any attempts to weave elaborate backstories into the narrative. This tends to mean that characters that do develop originality do so by surviving and being successful, earning originality as they make their mark on the world through their in-game actions.
Conceit #2: Discovery Through Interaction, Not Dice.
In more traditional games, it is typical for players to roll to determine if they spot traps, a secret door, or an enemy waiting in ambush. This is frowned upon in many OSR games, because secrecy and discovery are some of the main driving forces of the play style. As a result, you are expected to investigate places and objects via describing your character's actions, using critical thinking and common sense to uncover things and avoid hazards. For some games with a different focus this would be considered a waste of time, but sometimes a lack of mechanical focus tells you what a game or play style wants to spend time on.
Conceit #3: No Social Skills/Checks
Similar to the preceding point, characters are generally expected to talk to anything they come across, not roll a skill check to see how the social interaction went. This doesn't mean that everyone is talking in-character in funny voices at every OSR table (although sometimes this is true). In general the idea is to make players figure out the general premise of what their characters want to say and how they want to say it. This rewards player investigation into the people, places, and things of the world they are playing in. It also helps every player to feel like they can contribute to a conversation, not just the ones that have the attributes and skills for it.
Conceit #4: Game Master Neutrality
Those who run the game for the rest of the players are expected to act as neutral arbiters rather than be "fans of the characters" or "help facilitate great stories". This is difficult for many people to do, so there are various mechanics (see below) in many OSR games that are meant to help facilitate this neutrality. I don't really think true neutrality is possible, for the record, but using mechanics to determine things like how Monsters initially feel about you or when they flee battle go a long way towards achieving it.
Conceit #5 Combat Should be Avoided or Fought Unfairly
A frequent statement that gets touted as a truism in OSR circles is "Combat is a fail state". This is not necessarily true, but it is true that if you can avoid a fight and get the thing you want, you should do that. Generally speaking, combat should only be fought when it seems like the only (or only reasonably expedient) way of getting something you want or need.
Even then, combat should be fought as unfairly as possible. Ambush, use fire-bombs, separate foes using false pretenses, etc. In many newer forms of D&D, combat can often seem to be the point of the game. In an OSR game, combat is just one of many methods of overcoming obstacles in the way of character success.
Mechanic #1: Roll-Under Attribute Check
Any time a game or adventure tells you to do a roll-under check, it is telling you to roll a d20 and try to get under (or exactly on) the number the attribute it is tied to. For instance, if a module says that PCs should roll a Strength check, a character with a Strength of 14 would be expected to roll a d20 and try and get a 14 or below. This may seem odd to people used to rolling high and adding modifiers, but it has the benefit of requiring no math and an instant understanding of the results of a roll (you don't have to pause and add or subtract modifiers). There are variations, such as counting a roll of exactly the attribute's number a failure or using xd6 instead of a d20 to modify difficulty, but the basic concept remains the same.
Mechanic #2: X-in-6 Chance
Another mechanic that is used frequently in adventures due to its prevalence in B/X D&D and the like. It is a way of letting a dice roll determine something that you don't want to leave purely to Game Master discretion. Essentially, whatever X is determines your chance of success. You would roll a d6 and if it lands within the range described you get a favorable outcome. A 3-in-6 chance, for instance, is essentially a coin flip. Whereas, a 1-in-6 would be very low odds of success and a 5-in-6 very high. I prefer it to have parity with roll-under and consider low results successful, but other people prefer to look at higher rolls more favorably (Cairn's Die of Fate is an example of this). It doesn't really matter as the statistical chance is the same.
In general, this is a great way to try and decide something that would benefit or harm the PCs without violating neutrality. Though, personally, I tend to like 3-in-6 chances like the Die of Fate for such situations because it gives both possibilities even odds.
Mechanic #3: Reaction Rolls
I said before that social interactions are never bypassed or sped through via rolling, but a GM typically determines someone's starting disposition by doing a Reaction roll, which is done by rolling 2d6 and comparing the results to a chart. Typically extreme results are less likely both because the most extreme positive and negative Reactions are typically assigned to a single number (12 and 2, respectively) and also because the use of two die will skew the average result towards the middle.
Reaction rolls are really important if you want to show PCs that they don't have to constantly fight anything and everything they encounter. This is important, because parties that fight everything that moves will quickly get in over their head and die horrific deaths.
Mechanic #4: Morale Checks
Another tool in the "maintaining neutrality" toolkit. NPCs roll these when certain conditions are met. Usually when enough NPCs on their side are defeated, they personally are under threat of death, or a situation created by the PCs is frightening enough to warrant checking to see if they should turn and run. It is typically rolled with 2d6 (though I have seen 1d12) and the result is compared to the NPC's Morale score. The higher the Morale, the less likely that NPC will flee.
This is a useful mechanic for both saving time (playing out the clean-up part of a battle is pretty boring), fairly deciding enemy behavior, and (perhaps most important of all) modeling the idea of running away being a viable option for PCs.
Conclusion
Hopefully, whether you are a potential player or referee, this has made things a little clearer. If you haven't tried this style of game before I encourage you to give it a shot. It isn't necessarily better than the other types of Tabletop RPGs that are commonly played, but it is different enough that it is important to establish proper expectations. Good luck, and remember to pack enough torches and rope.