Back in the days before the Age of Strife, humanity had whole planets dedicated to housing criminals, a fair number of them were barely habitable heavy gravity worlds (called scientifically "Super Earths") for some reason. Then the Eldar started creating Slaanesh and society fell apart and these prisons were left isolated. The convicts then got out as things broke down and proceeded to go nuts in orgies of violence and sex. For the next few thousand years these louts ate food and drank water and moonshine with large amounts of mercury and lead in them, were subject to unhealthy doses of radiation, engaged in plenty of incest, and leadership (and the ability to acquire mates and spread one's genes which goes with it) was determined by who was the best at beating others with lead pipes. As no Ogryn has ever been recorded that was intelligent enough to farm or hunt, let alone distill, exactly where their food, water, and moonshine came from is an open question. By the time the Emperor got around to reunifying human civilization, there were dozens of planets full of dim-witted, durable, strong, three-meter tall dudes called Ogryn, in defiance of both evolution's tendency to diverge and how it tends to solve problems related to gravity (whales are big and heavy because they don't need to fight as much gravity - much bigger and heavier than flying birds, for whom gravity is a constant concern).
Ogryns in the Imperium
Though the Imperium of Man typically destroys anything too divergent from the human form, the Ogryns' DNA were considered stable enough to classify as a new evolution, or Abhuman. The Imperial Guard has been using Ogryn as heavy shock troopers for millennia, helped by the Ogryns' massive strength, iron-hard loyalty to the Emperor, and literal mindlessness. Perhaps the most appealing aspect of commanding Ogryn is that their loyalty to the Emperor of Mankind is firmer than that of a Space Marine. The Ogryns believe that the Emperor has personally issued them every order (being worked down from the chain of command) and as a result they will follow orders without question to the best of their ability. Technically this is already Imperial Guard policy to some degree, but only Ogryns actually believe it's literally true. The only reason that they aren't used even more is that they don't take well to transports - on top of their size, they have a tendency to get claustrophobic and aren't likely to willingly enter cramped, dark spaces (e.g. the inside of a Chimera).
Ogryns are usually used as close quarters soldiers; due to their physical dimensions it is very difficult for Ogryns operate human-sized weapons and equipment, even if they could understand it's operation, and are prone to becoming over enthusiastic or frustrated with equipment they are given, meaning that they tend to use objects in a brutal manner. Therefore they are issued heavily built, automatic shotguns that are essentially "Ogryn-Proof" and can be used as a club if the Ogryn ever runs out of ammunition, as well as oversized grenades. Since they are stronger they can also wear more armor than a regular human guardsman, but, this being the Imperium, under no circumstances are they given sophisticated or expensive equipment, let alone the sort of close combat weaponry they would excel with. While your average guardsman may not be too hot in hand to hand combat with an Ork, an Ogryn can crush Orks into pulp and their feats of strength typically surpass even those of Space Marines.
Particularly intelligent Ogryn are given genetic enhancements called Bio Ogryn Neural Enhancement (BONE) and become BONE'EADs, effectively squad sergeants. These enhancements make the Ogryns even more intelligent, being able to remember names, count on their fingers and make tactical decisions beyond "KILL IT!". Thankfully, other Ogryns accustomise to them quite easily, and rather than having to force their way into command by beating up the alpha male of the pack, other Ogryns will follow the BONE'EADS because "they know best". Hilariously, some Bone'eads take the title to heart, and use the metal plated cybernetic implants as an improvised weapon; thankfully, as with all things made for Ogryn, the implants are manufactured to take a bashing.
Some Ogryns are instead given Ogryn-sized tower shields and custom-built carapace armor which incorporates loose tank tracks; known as "Bullgryns" (copyrightable shorthand for bulwark ogryns), these Ogryns serve as living, mobile bunker walls on the battlefield. The idea is that all of that armor, behind a huge-ass shield designed to be interlocked with the neighboring shields, and with a big strong Ogryn inside of it, will soak up bullets, lasers, and other incoming nastiness that might hit more important things. It's a pretty sound theory. Each Bullgryn also carries an arm-mounted grenade launcher for fighting back with (though, being Ogryn-sized, it performs more like a handheld mortar). Instead of slabshields and grenade gauntlets, some Bullgryns are instead issued with power mauls and deflector bucklers (which generate invulnerable saves in the form of force fields) that double up as a makeshift bludgeon;
this being the Imperium, no Bullgryn is ever issued a maul and a slabshield, as of 8e Bullgryns may now be equipped with slabshields and mauls, slabshields and grenade gauntlets, deflector bucklers and mauls, or bucklers and gauntlets; however they may not take two mauls at once. And issuing both a slabshield and a deflector buckler to the guy whose only job is to stay alive while getting shot is heresy.
Ogryn Charonites are a type of heavy assault Ogryn troops used by the Solar Auxilia during the Great Crusade and Horus Heresy. Charonite squads have been biochemically and cybernetically altered using ancient technologies, possibly of xenos origin, that have been deemed unholy by the Adeptus Mechanicus. Because of this they are highly controversial among both the Inquisition and Mechanicus as they thread a fine line between loyalty and out right Heresy. With the Ogryns' hands and forearms replaced by a brutal collection of mechanisms known as Charonite claws, which amplify their already fearsome strength, Ogryn Charonite squads can rip through armor plating with ease, devastating their foes in the brutal confines of boarding actions.
Generally speaking, Ogryns are Auxilia forces to the Imperial Guard; having regiments on paper, but usually getting broken up and assigned to other Guard regiments on a squad-by-squad basis. But the Imperium does occasionally utilize entire formations of Ogryns: On the smaller scale, a Rampant Detachment is a platoon-sized formation of Bulgryns which can be trusted to hold the line and break up enemy assaults and is assigned its own dedicated junior officer to command them.
Other detachments that get deployed from place to place include "Abhuman Penal Suppression Detachments", which is a long-winded way of saying prison wardens or riot police. For example, the 51st detachment is made up of those Ogryns deemed "unfit" for frontline duties (either by injury or due to lack of intelligence even by Ogryn standards), and have been assigned positions on Imperial Naval vessels keeping troublemakers in line. The practice has since been taken up across other vessels and Imperial Guard bases where they have been used to quell riots among the local populations.
At the larger end of the scale, whole Regiments of Ogryns have been deployed in their entirety, with the Monglor Auxilia being deployed to Armageddon, which comparatively were considered "less brutal" than the Ogryns from Krourk, though only one Monglor regiment remains in active service since the Eldar caused their homeworld to turn renegade.
One other regiment includes the Orcan Stonecrushers which were enthusiastically integrated into the Imperium, but since their world had such oppressively high gravity it was turned into a Penal World and the Ogryns were permitted to be the custodians.
Gav and Bob - A story about some of the best Ogryns that gave their lives for the Emprah.
- Nork Deddog - THE most famous of Ogryns