The quantum oger has a pretty bad reputation. It’s the idea that as GM, you have made the decision that an encounter is going to happen, no matter where the players will choose to go. You present the players with two doors, one of which will have an angry oger behind it and the other a treasure. But you have also decided that the players should encounter the oger first and only after that find the treasure, because that feels more dramatically appropriate. As the joke goes, the oger and the treasure exist in a state of quantum superposition and their actual locations are not defined until the players open a door to look. Except unlike with real quantum superposition, the actual locations are not purely random, but the deliberate choice of the GM who wants to make certain events happen in certain ways regardless of what the players do. While in theory that could lead to adventures with great pacing that has the players under the illusion that things happen because of their choices, in practice players can read the signs of what’s going on in a GMs head and can spot patterns of things just happening at dramatically opportune moments. And once players get a hunch that their agency has been nothing but a lie, there’s little reason for them to care about continuing to play.
However, there is something very attractive about rolling random encounters in advance and spending some time on preparation to make the encounters something more interesting than “it attacks”. When you roll wandering monsters on the spot after a wandering monster check has determined an encounter, there’s already some amount of work to roll what creatures it will be, how many of them, whether the party of the creatures are getting surprised, what the creature’s reaction is to the party, and at what distance the encounter starts. This takes some time in which the players are waiting expectantly, which creates additional pressure to make the encounter start playing out quickly, and in such situation there’s always a strong instinct to just go with the default option of having them attack. Rolling the encounter in advance lets you put more care into all of it and in theory create more memorable scenes. The problem with that approach is that if you present players with a ready made encounter, it just doesn’t feel any different from an encounter that was written into a script by the GM. It doesn’t seem random at all, even if the GM tells the players that everything was totally rolled with no fudging.
There is a middle way, though, that combines the best aspects of randomly generated and placed encounters and advanced preparation while avoiding most of the main shortcomings: Roll the variables for the creatures that are being encountered, like type, number, reaction, and surprise, in advance, but determine neither a time nor a place when that encounter will take place. During play, at the end of each turn or when the players do something that could draw attention, make a wandering monsters roll. Roll the die in the open or let a player make the roll, with the players being told in advance what the numbers mean. (For an X-in-Y chance, I’m a huge proponent on of “something always happens on a 1!” Easy to remember.)
This is Heisenberg’s Uncertainty Oger. The type of the encounter is known, but it’s location and timing is not.
By making the wandering monster check in the open, the players have clear evidence that the GM had no part in deciding that this moment or this current location would be a great point to interrupt the party with a confrontation. Hostile monsters don’t kick the PCs when they are down because the GM thinks its funny. Neither do friendly NPCs appear in just the right moment to save the PCs asses. It was randomly determined that the encounter would happen in this room, at this moment. The pacing of the adventure is something entirely in the hands of players and the dice. No point in trying to predict what the GM might want to happen. There’s still of course the possibility that the GM could chose which of several prepared encounters gets used at any given point, but if that happens to be an issue you can write the creatures, numbers, reaction, and surprise on cards and put the stack out in the open where the dice are rolled, to have a player flip over the one at the top. You could shuffle them when you start playing, but there’s also an advantage to arranging them by hand.
The main reason why I want to do wandering monsters like this is that I find it very useful to know what the next encounter will be. This gives me an opportunity to spend some thought on how I would run the encounter if it appears in one of the two rooms that the players might go in next. For example, I know that the next encounter will be an ambush by bandits, and with that knowledge I can consider where I would put the bandits if that ambush is in the next room. There’s often not much time to spend on such thoughts while following what the players are discussing in the current room and answering their questions, but I still find it a lot better than being caught completely by surprise when the dice determine an encounter. If you have four or five encounters prepared and it could be any of them in any of the three rooms the players could decide to enter next, this just isn’t going to be possible.
Another neat thing that this approach helps with is encounters in which the players get surprised and the creatures don’t charge at them immediately. The original D&D rules don’t seem to consider this possibility at all, but when a creature gets surprised, I really like the option for those creatures quickly hiding behind cover and watching the party. The creature might even follow them around to observe, especially when the initial reaction is “uncertain”, or stalk them to wait for the best moment to strike. When you know in advance that the next random encounter will be such a situation, you can look at the players’ actions from the perspective of the observing monster. The random encounter check does not determine at what point the creature discovers the party, but rather at what point it will reveal itself to the players. This doesn’t really work when you roll all the parameters of the encounter after the random encounter check, because the players will know that you rolled several dice and looked up several things before announcing “you encounter nothing”. Having a random roll decide when a creature will show itself isn’t quite as good as really waiting for the best opportunity, but that seems to be a necessary compromise to have this kind of situation while still having the players see the rolls that determine when encounters happen.