When any monster is encountered, wandering or room-keyed, you must roll Monster Reactions. You can always choose the reaction, but rolling is the way to do it if you don't feel like choosing the reaction.
MONSTER ACTIONS: Some monsters always act in the same way (such as zombies, who always attack). However, the reactions of most monsters are not always the same. The DM can always choose the monster's reactions to fit the dungeon, but if he decides not to do this, a DM may use the reaction table below to determine the monster's reactions (roll 2d6).
Dungeons & Dragons Basic Rules, Moldvay edition, 1980, page B24
And the 2d6 table works like this:
2 Immediate attack3-5 Hostile, possible attack6-8 Uncertain, monster confused9-11 No attack, monster leaves or considers offers12 Enthusiastic friendship
So, 28% of the time, monsters attack or are hostile. If you draw steel, they attack. Remember, a 3-5 is still a possible attack; they are hostile, but there is a chance of negotiation. The other 72% of the time? Well, they will attack if attacked, but they are either indifferent, confused, or would consider teaming up with you 3% of the time.
On any of these, they will attack if you start swinging swords and flinging spells. Remember, the DM can choose anything from attack to friendly, and common sense applies! If the characters were killing orcs throughout the entire fort, it is highly unlikely that any of the ones left would be that friendly, unless they were prisoners from another tribe or so unhappy with the warchief that they just want a chance to get out of there alive. If they are part of the warchief's elite guards, they will attack 99% of the time.
Roll anyway to see if something interesting comes up, and then decide if it makes sense.
This table is another key part of old-school play, and you do not need to switch rule systems to use it. Yes, playing BX is cool, but we don't need to change the game we are playing in to make it feel more old-school! If we like it, we can keep it. The book says "Dungeons & Dragons" on the cover, so don't feel bad about adopting this as an optional rule for your 5E games.
Note, the Monster Reactions table will likely mess with published adventures, overall making them easier since not all encounters will be fights, unless you choose them to be fights. You may see the party getting an unxelected ally of the ogre in room 8, and he follows along and smashes the rest of the monsters in the dungeon he is finally sick of, and the party on his side finally gives him the bravery to say "enough is enough" and take revenge for the abuse he has endured over the years. That extra help from the party is going to trash the rest of the encounters on this level, but wow, what a cool story.
Why are you missing out on this gameplay? This happened all the time back in the day, and it's partly why BX is considered a legendary game. You can do this in whatever version of 5E you love, and there is no need to get your table to switch games.
As for that ogre making things too easy, this is another thing the old-school DM does. You may want to up the challenge in a later room, and you may want a reason for the ogre to walk away and head to the hills after they leave the dungeon. The town likely won't accept an ogre, and he will need to return to his tribe. Maybe he walks away in a bittersweet moment, and perhaps he could show up again in some later adventure as a surprise friend to help out in a time of need.
Again, this stuff happened all the time in old-school games. The game wasn't just "characters and combat" but a mini-sandbox game the DM could have fun with. We had rules like this to surprise even the DM, and this emergent gameplay can be directly dropped into your 5E game.
Just use this rule; it makes your 5E game feel more old-school and is fun!
