He has trained all his life to be a healer of the Line of Durin.
He had been a beardling during the Battle of Azanulbizar, his first true experience in war. He should not have gone, especially given his mother's reservations - but that was before she had spoken with Ferkin, Oin's mentor.
"You did not hear it from me," Ferkin rumbles quietly to her, with Oin listening with wide eyes, "but King Thror is a fool to try and reclaim Khazad-Dum. Especially given the past few years—" this is a reference to the overwhelming Dwarven deaths due to lack of decent food, shelter, and clothing, along with the terrible illness that had stolen away a quarter of the adult population and half of the children. "We will need all the help we can get," Ferkin finishes, looking sidelong at Oin. The apprentice is not yet old enough to understand the full impact of his statement, only that there will be a lot of people needing healing. It will not be until he is up to his elbows in blood, guts, and vomit, or dodging the uncountable corpses in between the tents, that he realizes the terrible truth of his profession.
Of course, nothing could have been done about Thror, beheaded as he was. Oin is glad for the small mercy of missing that sight. But Prince Thrain has gone missing, and thousands are dead, dying, or gravely injured. Prince Thorin, it turns out, had a shattered forearm, and Oin is fetching bandages while the healers order the prince to keep still, Your Highness, or you'll make it worse. The prince, of course, has no interest in doing as he is told (a trait that will follow him to his death), and so Oin also learns that when gentleness fails, a healer should not - could not - be above using more forceful tactics.
By the time a small group of travelling Dwarves, among them Thorin's younger brother, Frerin, is attacked by Orcs between the Shire and the Dwarven settlement in Ered Luin, Oin has taken on the responsibility of chief healer. He is still very young, far too young by traditional standards, but the few healers they have left are not young or strong enough to travel as needed - in fact, they get sick from such journeys more often than their actual patients. They are grumpy and whiny, and teach Oin from the warmth of their small healers' house, grumbling and shouting at him in varying levels of irritation and exasperation. Oin has learned to ignore their insults, as they are usually caused by pain and frustration, and listens attentively when they have something to teach. He fears he will not be able to hear once he grows old, too.
Frerin is barely alive when Oin rushes to the royal house, supplies in hand. He has an apprentice, of sorts, though she helps him carry things more than she actually learns. He spends days and nights by Frerin's bedside, cleaning and reapplying bandages, using every remedy known to Dwarfkind. He sends word to the Shire, hoping that their healers may have something to use; it is already winter, and he does not expect much, but he receives a small basket of herbs from them, and, more importantly, a precious jar of honey, with the instructions "use generously". He does so, applying the honey directly on the wound in hopes of stemming blood flow and keeping infection out. Frerin shivers with cold and sweats with fever, sometimes alternating, sometimes at the same time. He lashes out in bed and must be held down; sometimes he vomits and must be cleaned. Dis is almost always by his side, and Thorin too, usually - the siblings look exhausted and worried, as expected, for they have the added burden of leadership on their shoulders. Oin assumes that Fundin and his sons are helping, until he learns that Fundin died defending Frerin. His uncle's death rattles Oin, but he steels his nerve for Frerin and pretends the news is untrue.
In the end, none of his efforts matter. After nearly two weeks of fighting, Frerin falls into a peaceful sleep - one that worries Oin, because the prince does not look any better. Within hours, his last breath exits his body, and Oin tiredly draws a sheet over Frerin's face.
The settlement mourns heavily. Frerin and Fundin had both been beloved: Fundin, a protector of the people; Frerin, a cheery, light-hearted Dwarf that could turn the most sour of expressions upside-down. Oin's father, Groin, supports Thorin and his sister while dealing with his own grief, and Gloin stands by them as well.
Oin grieves alone. He had failed - failed in his biggest role yet, failed to save a member of the line of Durin. He grieves as a doctor, and a nephew, and a friend. He doubts his worth and his skills, even as the logical part of his mind tells him there was nothing more he could have done. Frerin had simply lost too much blood, and there was no cure for that. It is a guilt he will carry with him for the rest of his life.
His next challenge comes during a particularly bitter winter. It is not known where the illness comes from, but it is serious - deadly to any Dwarf that comes near it, if not treated quickly and properly.
Oin is one of the first to contract it. He fights to give himself the proper remedies for his symptoms, and only barely manages to keep himself alive long enough to fight off the disease. Fortunately, it means that he can work with the patients without fear, and he does so ruthlessly. But all the efforts in the world only save a bare few, and by the time spring rolls around, the hills are bursting with fresh tombs.
Oin is gradually learning the benefits of ale. The second child of Dis Thrainsdaughter will never know his father.
He cannot save Kili.
The foolish boy had gotten himself injured, not by just any arrow, no, but an arrow that appears to be made of Morgul metal. Even if they manage to find the Kingsfoil to counteract the poison, such an herb is not enough - not without an incredibly gifted healer, the likes of which can only be found among Elf-kind.
Oin would bang his fist against the table if he were not so preoccupied with Kili's feverish state. The sudden appearance of Orcs does not improve his mood, and the next few minutes are a blur of flying dishes, furniture, and bodies. Only after the last of the Orcs is gone does he register the presence of the Elf guard.
He watches her heal Kili with flowing words and athelas, wondering at the sight. Yet a small, niggling part of him reminds him that the prince - like his father, like his deceased uncle - would have died if only Oin had tried to heal him. For once, he thinks, he would like to be able to fulfill his duty as the royal healer.
He is woken by rough shaking, and cracks his eyes open to see a bushy red beard and worried brown eyes above him. Night is already settling, and for a moment Oin can hear nothing, nor see properly. Have I gone fully deaf? he wonders, until Gloin's voice slowly dissipates the fog in his brain.
Oin self-diagnoses himself with a mild concussion, likely from the large club that had caught him upside the head. Gloin hauls him to his feet before embracing him fiercely, and Oin reciprocates, confused. When Gloin finally pulls away, his eyes are red, and tears begin to run down his face.
Oin tries to ask what is wrong, but Gloin shakes his head and begins walking his unsteady brother towards the cluster of lights in the distance - tents. They step carefully around the bodies of Orc, Elf, Man, Dwarf, and Warg alike, and Oin apologizes to each of the Free People. I'm sorry, he thinks. I'm sorry I couldn't save you. He know - he knows - it is not his fault, but it is a mantra that runs through his mind every time he sees death.
He sees Bilbo hunched on a log, and there is nothing silent about the Hobbit's grief. Oin stands outside the royal tent and fears - what will he see? What could make Bilbo so broken?
Three figures lie still on the cots in front of him. Fili and Kili rest off to the side, their cots pushed together, their hands touching. They could be sleeping - but there is blood, too much blood, and they are too pale. And Thorin...the once regal King lies, head propped, on the cot directly in front of him. His eyes are closed, his bloodied hair spread out around his head, and a multitude of cuts mar the sharp features of his face. Dented, wrecked armor rests off to the left of the cot, covered in red blood. The bodies are the same size, and yet so much smaller; the life in them gone, and taking their size with them. Perhaps that is why they build enormous statues, Oin thinks. To emphasize the large life held in such small forms.
Oin looks at Fili and Kili: fine Dwarves that he had known, quite literally, since birth. He had patched up every bruise, scrape, and broken bone they had ever had, fed them soup and potions whenever they were sick, forced them back into bed when they wanted to be up and about.
He had known Thorin since that fateful day, when the screams of the dying were all around them as he held the stubborn prince down and watched his teacher expertly heal the wounded Dwarf.
He was the healer, the caretaker, the one with the gruff demeanor and the terrible bedside manner. He has fought and held back death, sometimes, but he has too often let it win. He was supposed to watch over and heal the Line of Durin, and in the end, his efforts have amounted to nothing.
It is not silent after war, and Oin wishes he were completely deaf.