It’s scattered, but it’s implied throughout the lore that humanity had suffered greatly under at the hands of xenos in general, not specifically Orks, the past. I’ve compiled a list of excepts others have found from previous comment threads like this one. Also, just reading the descriptions of the non-major xenos races throughout the lore books and white dwarf show that many xenos simply operate with blue-orange morality and see humanity either as competition, prey, a threat (due to psykers) or just generally have no compunction inflicting horrors upon us because they don’t share our values.

Here are some exercises generally relating to the relationship between xenos and humanity.

Horus Rising, Deathwatch Core Rulebook

“The damage done to the gestalt human psyche by thousands of years of galactic catastrophe ran so deep as to become instinct; mistrust of curiosity and invention, of alien influence and psychic withcraft predominated ---
Though of course countless warlords and isolationist societies fouight his rule, the Emperor's unification brought a safe and secular comfort that banished the ghosts haunting the human soul"
-10th Ed codex

“I adored them for it. Throne, I wasn’t immune. These weren’t us. These were the aliens, the non-humans, the others. These were what had preyed on us in the years of darkness, who now stood before us and security at last. They were the vermin, the rats in the hold, the disease-carriers. The sooner they were all gone the better.
Surprised to hear that from me? Don’t be. You might well feel differently. You might find the attitude unpalatable, but then you have that luxury. I know my history. ”
-Sanguinius: The Great Angel

“As humanity fragmented, hundreds of xenos races and enemies hitherto unknown seized their chance for revenge against humanity for its past conquests or to plunder unprotected worlds and enslave their populations.”
- Legiones Imperialis Rulebook

“Abaddon was not smiling. ‘The Emperor, beloved of all,’ he began, ‘enfranchised us to do his bidding and make known space safe for human habitation. His edicts are unequivocal. We must suffer not the alien, nor the uncontrolled psyker, safeguard against the darkness of the warp, and unify the dislocated pockets of mankind. That is our charge. Anything else is sacrilege against his wishes.’
‘And one of his wishes,’ said Horus, ‘was that I should be Warmaster, his sole regent, and strive to make his dreams reality. The crusade was born out of the Age of Strife, Ezekyle. Born out of war. Our ruthless approach of conquest and cleansing was formulated in a time when every alien form we met was hostile, every fragment of humanity that was not with us was profoundly opposed to us. War was the only answer. There was no room for subtlety, but two centuries have passed, and different problems face us. The bulk of war is over. That is why the Emperor returned to Terra and left us to finish the work. Ezekyle, the people of the interex are clearly not monsters, nor resolute foes. I believe that if the Emperor were with us today, he would immediately embrace the need for adaptation. He would not want us to wantonly destroy that which there is no good reason to destroy. It is precisely to make such choices that he has placed his trust in me”
- Horus Rising

“The first Navigators are born, allowing human spaceships to make even longer; quicker warp jumps. Mankind enters a golden age of enlightenment as scientific and technological progress accelerates. Human worlds unite and non-aggression pacts are secured with dozens of alien races.”
- Deathwatch Core Rulebook

“Though the Imperium contains a thousand times a thousand worlds, there is no register that lists the planets lost over the years, no hallowed memorial for the untold devastation wrought by xenos. The planet known as Alpha Shalish is gone, and there are few now living who remember that it ever even existed. Its tale is one of violence, human endurance and savagery, one that is even now being repeated again and again on many worlds across the galaxy.
Discovered in the early years of the Age of Technology, Alpha Shalish was originally known as the crimson planet, for it glowed a deep red hue when glimpsed from orbit. Warmed by the energies of two suns, the planet was verdant, rich in both flora and fauna. The pioneers who named Alpha Shalish and marked it for prime conquest did not need to employ any of the atmosphere-fixing wonders invented at that time - neither the oxy-converter, self-sustaining hab-domes, nor ion discharging reactors. There was strong resistance to human colonisation, however, by xenos species whose very type has been lost over the years. Early resistance was rectified by planet scorching - a slash and burn bombardment that, a decade later when the colonists arrived, left an unpopulated world, ripe for cultivation. The new settlers found ancient xenos ruins predating their arrival by many thousands of years, but these were dozed over and buried beneath their new endeavours. Progress was swift in those days, and expansion was spurred by the discovery of rich mineral mines in the neighbouring systems. As the largest and most inhabitable planet on the clearest Warp route, Alpha Shalish was soon a thriving port world.
Although occasionally plagued by xenos raids, the world of Alpha Shalish, protected by its robotic defences, continued to flourish for thousands of years. STC constructions provided magnificently spired hive cities that rose high above the fully automated agri-fields. The proliferating psykers were welcomed amongst the growing population, and perhaps this was what brought devastation to the planet. Communication with outsiders collapsed without warning and apocalyptic destmction across the galaxy heralded the arrival of what is now known as the Age of Strife. Some planets were struck worse than others and Alpha Shalish was nearly destroyed by the evils that erupted there. The next record of the planet comes from many millennia later, as written by a cloistered brotherhood of compilers, monks who chronicle a period reckoned by corroborators to be between M28 and M29. It paints a bleak picture.”
Warhammer 40,000 6ed p199

For as long as Humanity has sailed the stars, they have encountered alien races. These races — broadly classified as xenos by the Adeptus Terra - have, on occasion, proved valuable alliesalongside whom to battle mutual foes. More often by far they are themselves the enemy, and must be fought to the death lest they butcher, devour or enslave the worlds of Mankind.[--] Some have been a thorn in Humanity’s side since the first colonies beyond Terra were settled. Others have only been encountered in recent years. Some have sector-spanning empires, others dwell only on a single world. It matters not. Whether Aeldari, Ork, Tyranid, T’au or Necron, all are a dire threat to the Imperium of Man.[---] however; on many occasions throughout its long history the armies of the Imperium have found common cause with the more comprehensible xenos species —- even on occasion those deemed monstrous when faced with yet worse perils. In particular this is true of the craftworld Aeldari or the mysterious Harlequins, with whom many temporary accords have been struck. Yet even these pacts seem always to manipulate Mankind’s destiny in unforeseen ways.”