"FAHCK YOU, DIGGANOBZ!"
Digganobz were originally a group of Humans in the specialist game Gorkamorka, who had forgotten their Imperial lineage and were now influenced by the Orks, to the point where they cosplayed as Orks. The term has now come to refer to any non-Orks who for whatever reason have Species Dysphoria, and either want to be Orks or believe themselves to be Orks.
Reflecting that Orks in 40k were historically boldfaced references to British politics, the word digger is British slang for Australian soldiers, who have their own reputation for cunning brutality.
When the Orks crashed on Angelis there was a Human ship, implied to be an Adeptus Mechanicus exploration and research vessel, in orbit at the time, having already landed a mission on the planet. The cataclysm of the Space Hulk's impact separated the Humans into two factions, based on where they were at the time. Diggas're descended from those Humans who were exploring the pyramids and the caverns below; taking shelter under the planet's surface, they survived the holocaust above, but descended into primitivism.
Diggas have forgotten all about their ancestral connections to the Imperium and have come to revere Orks as the most awesome and impressive creatures they've ever seen. Therefore they try to emulate the Ork lifestyle as much as possible. Orks tolerate this with almost paternalistic disdain; it is, ultimately, much simpler than trying to wipe the Diggas out, especially since they've got the protection of "dem fings under da pyramids". The home of the Diggas is referred to as "Morgarg-Durlurk-Gulskar-Dregsnikslag", which translates as "Fortress of ancient, terrifying power. Land of waiting death, pain and destruction". This shows just how much dread the Orks have of the place. Besides, the Diggas happily trade technological gear dug out from under the pyramids and scrap they have gathered for good Orky stuff, and it's funny to watch them try and be Orks, so it's not so bad.
Throughout the galaxy, Orks have conquered and enslaved entire Imperial Hive Worlds, and put their considerable populations to work manufacturing arms and armour for the hordes. Subjected to arguably even worse conditions than before, it's common for some to break, and begin seeing their Ork masters as an inherently superior race, potentially producing entire regiments of Diggas. Whilst many Orks don't have the time of day for them, Blood Axes appreciate the ability of Diggas to act as double-agents, infiltrating Imperial institutions to gain access to shipping timetables and sabotage vital infrastructure like their own version of Genestealer Cults.
Diggas can be summarized as "hard mode Orks", essentially. Of the three "oddmobs" they're the simplest to use, but they suffer a number of penalties in comparison to their Orky counterparts. Inferior stats, a special rule that means vehicles will eventually break down, the fact that a Diggamob cannot visit Mektown (and thusly get injuries treated at the Dok's Serjery or have their vehicles tended to) until they've won a battle or survived two fights, greater problems getting gubbins (vehicle upgrades) fitted, and increased risk when visiting the Dok all make life as a Digga much tougher than life as an Ork. On the upside, their Shamans grant them access to a number of useful tricks and skills, their troops're all cheaper than Orks, and they can also pick up a number of powerful "ancient tech" devices. Unfortunately, these devices are extremely unpredictable, and put you at the mercy of the random number god.
On the Tabletop
Diggas never got a representation in the main game, but many players have used them as Counts As Catachan Imperial Guard allies, though obviously as of 8th Edition this is now only viable in Narrative and Open Play, as they don't share a Faction Keyword that'd let them join each other in Matched.
Another method would be to use human models as stand in for grots, they even have similar ballistic skill so it's not that farfetched as far as proxies go.
- Blue - A T'au woman who was mistaken for a Gretchin as a child, and adopted by the Scraplootas Warband.
- Clarence - A Human krewmember aboard Da Looted Krooza in the Deffwotch Rogue Trader (RPG) campaign.
- Herman von Strab - Another Human that ended up working for the Orks.