Old school play is associated, for good or ill, with lethality. Characters can and do die. Save-or-die is a thing. Encounter balance is at best a loose guideline, not a sacred entitlement. Some old school DMs will actually make you roll your hit points at 1st level, and a good many of us don't go in for any of that negative hp business, either: zero hp is dead, full stop. Some of us don't even allow spells of resurrection in our games. Dead is dead. Get out the dice and a fresh character sheet, because your old guy isn't coming back.
That's one of the complaints that new-school gamers have against the OSR: It's a meat grinder. Who wants to roll up character after character only to watch them die ten minutes after entering the dungeon? A deadly game and a survivable one don't have to be mutually exclusive, though. The buffer between fragile characters and the merciless jaws of monsters is player skill. Old school play is not merely matching character skill against dungeon hazards; it's matching character skill AND player skill against the monsters and traps in the dungeon. Choices made without information are random, and neither require nor benefit from player skill. Player skill requires information to make choices -- which means that danger needs to be foreshadowed.
Foreshadowing can be applied to entire dungeons or areas, as well as to specific locations within a dungeon or other adventure locale. With good foreshadowing, the players can decide to avoid some area if they believe there might be something there they can't handle. They may also prepare themselves to face whatever is there.
There are lots of ways to go about foreshadowing, and varying degrees of subtlety. Which ones you use, and how blatantly you apply them depends on the particular situation and the skill level of your players. Very young or new players will probably need more obvious clues, while long-time players may relish the challenge of a more difficult puzzle.
Rumors: Before the party even leaves town, they might pick up rumors of very dangerous creatures and places. Naturally, first-hand accounts are usually the most reliable and least distorted, but tavern tales and folklore have value as well. If the locals tell you that the Wraith King had every chest in his tomb trapped, or that the orcs have something huge and ravenous chained inside the old mine, or that legend says a demon is sealed beneath the abandoned wizard's tower, you might want to take heed.
Spoor: Direct physical evidence of a creature. This could be tracks, tooth or claw marks on logs or rocks, droppings, slime trails, shed hair or scales, scorch marks, or whatever signs a monster or group of monsters might leave in its wake or use to mark its territory.
Aftermath of an attack: Bodies, body parts, blood, or equipment and possessions of past victims. These might bear the marks of how they died for extra clues. Was it bitten clean in half and left behind? Bludgeoned to a pulpy mess? Parts of it eaten? Robbed or ritualistically disfigured? All clues for a clever party to interpret.
Behavior of other creatures: Birds, rats, insects, and other creatures may be aware of approaching danger before the PCs are. Maybe the party's own mounts and animal companions know something they don't. Do they flee, fall silent, perch all around and watch expectantly, panic, drop dead, chitter or cry warnings to each other, or something else? What if it's bears, wolves, or elephants flipping out instead of mice and sparrows?
Smells: Does the monster or its lair have a particular odor? Rotting carrion and brimstone are scents associated with peril, but anything that's different from the rest of the area may be a clue. Ozone, tar, stale sweat, smoke, acid, mineral spirits, pulverized stone, mold, fresh moist earth ....
Sounds: The chittering of an excited horde of kobolds, the labored breathing of a slumbering dragon, the click of claw on stone, heavy footfalls approaching from far down the corridor, ghostly whispers, howls and roars of rage, the screams of victims or prey ....
Messages: Inscriptions on sealed portals, vague warnings scrawled in chalk or charcoal, a forgotten adventurer's journal, a rat-gnawed map of the dungeon with something illegible scribbled in red, a magic mouth or phantasmal force, hash marks chiseled into the walls beside doors (a secret code) ....
Information from other dungeon dwellers: Maybe the goblins know what lives farther back in the caves, but you won't find out if you just slaughter them on sight.
Sixth sense: A bad feeling upon entering or approaching some place. Dreams or premonitions. Especially appropriate for creatures or places of a supernatural persuasion.
Changes in the dungeon: That twenty foot high door made of burnished bronze, when every other door in the dungeon is man-high and made of iron-bound wood ....
Of course, this is in no way intended to be an exhaustive list. When you describe details which foreshadow danger, it's a good idea to crank your verbal drama dial up a notch or two from your narration of less perilous dungeon features. A lot of your descriptions are simply background imagery, setting the scene, but foreshadowing is serious business and it's important that you cue your players in to the difference. Use scary and ominous words. Repeat yourself if you need to. Modulate your voice for maximum impact: an awed or sinister whisper, dramatic pauses, or whatever tone you feel best conveys the emotion of the moment. Leave no doubt in the players' minds that while you've described other rooms in the dungeon as rank and musty, the smell wafting up from that dark staircase is way beyond that.
For extra impact, double up on foreshadowing. For instance, the party may hear a story of a knight who bore the token of a sword thrust through an anvil, who was slain by an evil coven of hags. Then when they find an old rusty shield with the sword and anvil device, they'll know what it means, and what may be in store for them. Or perhaps they learn of the Wraith King who worshiped a god of unending life and sealed himself away in a vast room full of fabulous treasures, and then come upon a huge golden door deep in a shrine devoted to that god. Or they discover an adventurer's log with a drawing of a hideous monster accompanied by fevered ramblings of what it did to his companions, and then find granite columns bearing the slashes of huge, terrible claws.
Occasionally, it's OK to throw in a twist. Maybe things aren't exactly as local rumor has made them out to be. Perhaps the dragon is really just a gang of bandits stomping around with dragon-foot-shaped boots to discourage anyone from prowling the forest near their hideout, or the giant bellowing dire threats to intruders into the mountains is really just a goblin shouting through a hollow log. It's important not to do this too often, to reward the players with correct information if they investigate in a way that would reveal the deception, and for it to make sense in hindsight even if they are fooled.
In the end, if your players go charging into a killer encounter and get their party wiped out, it will more than likely be because they chose to take that risk, not because the DM sprang a "Gotcha!" on them that the dice couldn't save them from.