In Oblivion there are several vulnerable NPC's that without the player's doing will end up dead eventually, making the game a lot less lively.

This "mod" is just a simple .txt file for making NPC's that tend to travel ingame essential. I also covered some NPC's that tend to get themselves killed in other ways.

NOT QUEST-BREAKING!

Details:

The following NPC's I did NOT cover in this file
- NPC's that are supposed to die during quests (Unless mentioned otherwise, see 'Optional:' below).
- NPC's that are located in a safe environment.
- NPC's that respawn

The following NPC's ARE covered in this file:

Ongar the World-weary, City-Swimmer, Alessia Otus, Algot the Northerner, Ancus Afrianus, Carandial, Dar Jee, Luciana Galena, Orrin, Cicero Verus, Countess Alessia Caro, Dovyn Aren, Dralora Athram, Dro'Nahrahe, Dul gro-Shug, Elisa Pierrane, Elragail, Erthor, Geem Jasaiin, Gelephor, Hassiri, Hastrel Orrus, Herminia Cinna, Hlidara Mothril, Honmund, Ida Ottus, Ida Vlinorman, Jean-Pierre Lemonds, J'skar, Lerexus Callidus, Mandil, Marinus Catiotus, Margarte, Mirili Ulven, Oghash gra-Magul, Othrelos, Pennus Mallius, Pista Marillin, Praxedes Afrianus, Quill-Weave, Ra'Jhan, Ra'jiradh, Renee Geonette, Rochelle Bantien, Roderic Pierrane, Romana Faleria, Salomon Geonette, Samuel Bantien, S'rathad, Stantus Varrid, Tertius Favonius, Thamriel, Torbern, Toutius Sextius, Trenus Duronius, Undena Orethi, Urjabhi, Varnado, Winson, Wumeek, Yngvar Doom-sayer, Farwil Indarys, Bremman Senyan, Mazoga the orc, Brag gro-Khash, Rienna, Elidor, Elante of Alinor, Hill the Tall, Gruiand Garrana, Avrus Adas, Silana Blandia, Kantav Cheynoslin, Shagol gro-Bumph, Dar-ma, Malintus Ancrus, Dro'shanji, Uzol gro-Grulam, Dranas Llethro, Hillod the Outlaw, Isleif the Open Handed, Carwen, Dynari Amnis, Reman Broder, Glistel, Ra'qanar, Varon Vamori, Dro'shanji, Adanrel, Bronsila Kvinchal, Jair, Kastav Kvinchal, Vlanarus Kvinchal, Velan Andus, Shady Sam, Floyd Nathans, Hanz gro-Hubrag, Philip Franc, Astante, Merildor, Torbal the Sufficient, Aengvir, Deetum-Ja, S'mirra, Candice Corgine, Schlera Sestius, Shafaye, Aelwin Merowald, J'riska, M'dasha, M'dirr, S'thasa, Tsalajma, Tsramla, Zabhila, Brother Hjar, Brother Holger, Brother Hridi, Sister Angrond, Sister Phebe Jeanard, Jhared Strongblade, Brother Piner, Aloys Bincal, Callia Bincal, Corrick Northwode, Thorley Aethelred*, Uuras the Shepard*, Sakeepa*, Maeva the Buxom, Arielle Jurard, Roliand Hanus, Hirtel, Ilend Vonius, Astia Inventius, Honditar, Langley, Gerich Senarel, Tyrellius Logellus (original "Bruma dungeon" version), Amminus Gregori, Carmalo Truiand, Giovanni Civello, Decentius Opsius, Danus Artellian, Dion, Davide Surilie, Gaston Surilie, Tamika, Bernadette Peneles, Shelden, Mirili Ulven, Runs-In-Circles, Bruscus Dannus, Zoe Malene, Tall-Trees-Falling, Pyke, Halion, Relan, Ranarr-Jo, Kishashi, Cindanwe, Erver Devani, Dulphumph gro-Urgash, Beelei, Wide-Eye, Gundlar, Nelrene, Kithlan, Herdir, Anya Herrick, Ushnar gro-Shadborgob, Una Armina, Sickly Bernice, Ma'zaddha, Gloorolros, Earil, Cutter, Caldana Monrius, Brithaur, Bolwing, Bhisha, Arctus, Ahjazda, Uungor, Tove the Unrestful, Tilse Areleth, Thaedil, Sontaire, Sheer Meedish, Rendil Drarara, Raven Biter, Pad-Ei, Orinthal, Fimmion, Dumag gro-Bonk, Dervenin, Big Head, Amiable Fanriene

*along with livestock

Other characters:

Black horse courier horses (the couriers themselves respawn, only their horses don't)

All of the Beggars

All of the stable workers

All of the worshippers at the Daedric Shrines

All of the Kvatch Soldiers

All of the Master Trainers

All of the Bruma Allies during Defence of Bruma (Including Viera Lerus, excluding Ulrich Leland)

Jared Ice-veins (will stop being essential after his quest ends and will start roaming around the Shivering Isles, just enter the command again for him to remain essential for the rest of the game) 

Baurus (will stop being essential after The Path of Dawn quest. Just enter the command again after the quest for him to remain essential for the rest of the game)

Jauffre (like Baurus he will stop being essential at some point during the main quest. Just enter the command again when this happens)

All of the Daedric shrine questgivers (Will stop being essential after completing their respective quests, like above just enter the command again)

Berich Inian (One of the Kvatch soldiers, will stop being essential after stage 40 of The Battle for Castle Kvatch quest, like above just enter the command again)

Methredhel (will stop being essential after completion of The Ultimate Heist quest, like above just enter the command again)

Armand Christophe (see above)

Amusei (see above)

S'krivva (see above)

Ahdarji (will stop being essential after Ahdarji's Heirloom is completed, like above just enter the command again)

Larthjar the Laggard (one of the beggars, will stop being essential after Lost Histories is completed, like above just enter the command again)

Melus Petilius (will stop being essential after Molag Bal's Daedric Shrine Quest is started, like above just enter the command again)

Drarana Thelis (will stop being essential after Mystery at Harlun's Watch is completed, like above just enter the command again)

Irroke the Wide (will stop being essential after Clavicus Vile's Daedric Quest is completed, like above just enter the command again)

Ri'Bassa (will stop being essential after Sheogorath's Daedric Shrine Quest is completed, like above just enter the command again)

Eronor (will stop being essential after Find the Heir is completed, like above just enter the command again)

Barthel Gernand (will stop being essential after Goblin Trouble is completed, like above just enter the command again)

Burd (will stop being essential after Defence of Bruma has begun, like above just enter the command again)

Carius Runellius (will stop being essential after A Brotherhood Betrayed is completed, like above just enter the command again)

Bittneld the Curse-Bringer (will stop being essential after Canvas the Castle is completed, like above just enter the command again)

Hieronymus Lex (will stop being essential after Taking Care of Lex is completed, like above just enter the command again)

Itius Hayn (will stop being essential after Imperial Corruption is completed, like above just enter the command again)

Valus Odiil (will stop being essential after The Killing Field is completed, like above just enter the command again)

The Odiil brothers (will stop being essential after The Killing Field has begun, like above just enter the command again)

The Jemane brothers (will stop being essential after Sins of the Father is completed, like above just enter the command again)

Ulen Athram (is actually a Mythic Dawn sleeper agent, however it is quite easy to avoid the sleeper agents all-together. If avoided these sleeper agents will stop being hostile when the main quest is completed and they will resume their normal lives. If you want Ulen Athram to be killable anyway just enter the following command in the console: setessential 1fc55 0)

Install:

1. Put essentials.txt in your Oblivion directory
2. Ingame press "~" to enter the console
3. Insert the following command: bat essentials.txt

Optional:

Should you want to kill the 3 suspects in the Paranoia questline:
1. Put paranoia.txt in your Oblivion directory
2. Ingame press "~" to enter the console
3. Insert the following command: bat paranoia.txt

If I'm missing any vulnerable characters, please let me know!