The players should have already met. How is left up to the GM. There should probably be some events leading up to this. Probably something along the lines of increased raids by bandits and/or monsters in the area. Here's the set up:
As the players are on the road, they come across the charred remains of a caravan. A merchant is still alive, though barely, and mutters the word "Ogres". Little do the players know that the ogres are still around. As the players prepare to leave, deciding there probably isn't a whole lot they can do, the band attacks again. It consists of 2 ogres or trolls, 3 orcs, and 1d4 goblins. They are all equipped with leather armor and short swords (long swords for the ogres or trolls).
At least one of the monsters should be allowed to escape, leading the party back to their camp (that's right, camp, not lair). The camp consists of several hundred of the monsters, all conducting raids in full cooperation. Obviously they are under the influence of some greater being. And any being able to control this many members of the monster races in one area without a bunch of infighting must be powerful indeed. This could lead to entire kingdoms falling should the army be mobilized.
What the creature is that is controlling/commanding the monster races is left up to the GM. If you want, you could go the route of "The Demon Awakens" (a great novel by Robert Salvatore, but not AD&D related) and have them controlled by a baalrog or other greater demon of Hades. Another option is a dragon, hatchling (though not likely) or adult, or other similar being of greater power. If you really want to scare your players, you could even make it a member of one the monster races (probably an ogre or goblin cobbler) that has taken to the study of magic. Players always underestimate the monster races, especially goblins, for some reason.