Errantas
In my Errantas homebrew (2 campaigns set 80 years apart), all of the races (except effectively underdark Halflings) were brought to this material plane as a refuge because it had thin elemental walls. (Custom cosmology - just basially it was easier to get to.)
So I had orcs that orginated from two different planes in my world. The Steppes Tribes were loosely based on Ghegis Khan and the mongols, had seven tribes each with some different core beliefs, plus another tribe that retreated before start of play into the underdark, taking with it a seedling from the mother tree of a CN druid grove that helped them adapt. One of the tribes became allies becasue of the action of the party and controlled much of the steppes by the second campaign.
The other group was the Archipelago Orcs, who were fearsome sailors and were the strongest naval power around. They were slavers as well. It wasn't explored in play, but they were beholden to something demonic in a volcano.
13th Age
Last finished campaign was 13th Age. The Orcs had long ago been created by the Elves to defeat the dreaded Wizard King (now the less powerful Lich King), who then lost control. Various other races inhabited the lands they allied with, but the races were showing to grow together, especially in Concord and other cities, which threatened the Elf Queen's rule over the elves. So she set up a new Orc Lord by breeding a new, more powerful one and sending a clandestine champion to help him unite the orc tribes under him. (This is the work of of decades.) She also captured a living dungeon and started a decade long ritual to force feed it the evil bubbling up from below when the Drow tainted the underworld to win their war against the dwarves. She knew that living dungeons were what you got when the taint infected new Koru Behemoths as they swam through the earth to the surface, and wanted to make a crazed Behemoth that would wipe Horizon, the floating city of the Archmage off the map, and then go one and wipe out Axis, the capital of the human's empire. The entire city of Horizon, she had found out, was the phylactery of the Lich King.
The Orc Lord, having been created stronger, wasn't just a pawn. He used the elven magic on sacks of orcs gestating beneith the earth (orcs bubble to the surface in waves of evil, as well as being born) to create elf-enhanced orcs.
So the campaign spread the time from right before the Orcs started their attack to until the Dwarves had been contained and the Orc armies were encircling Horizon and starting the ritual to guide the living dungeon/behemoth there. The campaign started and ended with the Orcs and their plans, though it circled the entire map to do so.
Current
In one of my current campaigns, the players are agents for a fading and failing Imperium, well past it's peak. The continent has been mostly civilized, and orcs are not a part of it. There's no indication in play yet if they never existed, or if they are extinct like dragons and dwarves are.
There's also a possibility that I may introduce them later - both the drow and halflings are races created by the Imperium at it's peak, and I could have Orcs be a new warrior race being created for the war that is brewing.