TLDR: Make a world. Put interesting stuff in it. Some stuff is really bad. Some stuff is really good. It’s hard to tell the difference until characters start messing with them. The DM and players tell the stories, not you.
Before I puff myself up as an authority and thus put myself out there for ridicule, let me start with the most controversial aspect of OSR adventure design.
It’s Not Your Game.
If you’re designing for the OSR, the first thing you have to realize is that the game does not belong to you. It belongs to the DM, the players and their characters. The DM is going to take your carefully crafted magical thing and hammer it to fit with an encounter from Against the Giants, and a random table from the d30 Sandbox Companion, a neighborhood generator from Metal Gods of Ur-Hadad, an NPC based on Private Hudson from Aliens, and a thousand other things of the DM’s own design. And when the players guide their characters into Undersewers of the Mole Gods, they are going to take your perfectly balanced, story-relevant encounters, ball them up, shake out all the XP and gold, and pitch them over their shoulders. All that’s going to be left is the story that the players and the DM tell years later, swigging beers at the Ram Brewery after their last con game. You get part of that story, but you don’t get to tell it.
Have Lots of Things for the Murderhoboes to Mess With
The OSR adventure is not a pre-scripted adventure path with a natural story arc balancing every character role into session-digestible chunks. An OSR adventure is a collection of evocative structures thrown down on a map, some with connections, and others just hanging out there. Your job, as a designer (and I believe it’s your only job) is to build the structures that the characters and DM can pour their stories into. Some of these structures have solid foundations, like an NPC with a detailed backstory, concrete motivations and secret weaknesses. Some may be paper-thin props that when viewed from another angle are nothing more than a blanket fort. Designers use this all the time – an evocative sentence or two to describe a building in a town or a locale on an overland hexmap.
The point is, the characters need stuff to mess with (and typically to destroy). Can there be connections? Sure! There’s a spy in town investigating the cult that gathers at a nearby dolmen every dark-of-moon. Why is the spy there? Who does she work for? Where is her safe house? These questions help to build further structures, thin or solid, but they are not steps in a process for “solving the story.” All of these structures coalesce into a mini-world that may be as small as the town where the spy resides or as large as a galaxy.
Some of the Things are Horrifyingly Bad. Some of the Things Are Amazingly Good. It’s Impossible to Tell the Difference.
Players aren't interested unless their characters are simultaneously threatened with horrible death and unimaginable wealth and power. And they aren't going to stay interested if they can tell the difference.
The bad things need to appear to be horrifyingly bad. Do you throw up “Goblins (3); rusty short swords” or eyes that gleam in the darkness, faint gibbering and the scrape of metal on stone? What are we fighting? What’s its “power level?” Is there any benefit to pitting my beloved Ussa-La the Space Princess against this unknown danger? Do I take the shiny without checking for traps? Or do I risk taking the time to be careful with the constant threat of another horror coming around the corner and trying to eat me?
Piles of treasure, eldritch artifacts, a shiny new space ship, level up… All of these things drive players to put their characters into terribly dangerous situations. And, they’ll do the same thing just for a rumor of these things. A grizzled yazirian holding court in a dusty cantina swears that the UPF ditched a super-secret spy ship on the prison planet of Holeefuckdontcomehere 9S. Not only is it packed with amazingly cool gear, there’s a case with a million credits stowed in a weapons locker. The planet? Nah! I’m sure it’s mostly deserted.
As a designer, make every fight a trepidation. Make running away regretful. You are not in the business of balance. Rewards are not parceled out in commiserate-with-dangers-engaged precision. Uncertainty. Every threat need not be insurmountable, but the threat should appear to be significant. Uncertainty.
And some of the threats should be insurmountable, especially with the stats and things on the player’s character sheet. Some rewards should be wildly overpowering and “unbalancing” to the game.
It is uncertainty that adds all the tension to the game. It is player ingenuity that leads to its greatest triumphs. And both of these keep the players coming back for more. There’s something amazing out there, and even though there may be horrifyingly bad things guarding it and even though the amazing thing may not be as awesome as rumored, I will go into this world and find out.
Your only job is to build that world.