Warning: vanilla Oblivion does not play nicely with large texture packs due to poor memory management. Texture memory patch is highly recommended for this mod or you might experience CTDs. One option is to use ORC with this preset. Also, >4GB VRAM recommended. 4x upscale = 16x VRAM footprint.
Also, I recommend to look through the "Core" section of this guide for other essential bugfixes.

Better Cities upscaled textures
It is a mod, well, with ~2300 2x/4x upscaled textures (along with normal maps) for BC, covering the majority of the mod's textures (excluding LODs, character faces, icons, some technical lowres textures and a few textures which are clearly not improved by upscaling like stuff with printed text). B&M/CUO versions of Bravil have also been upscaled. The approach taken falls between OUT's batch-upscaling with single model and Morrowind's MET meticulous manual experimentation with each texture with the result hopefully being improved visual fidelity. The mod is somewhat in the WIP state: 4x upscale is an overkill for Oblivion for many of the textures, so a reduced version is in plans. Texture size is already capped at 4k, and some 4k normal maps have been scaled down to 2x, but further optimization is still required.

Process description
1) Original textures cleaned and decompressed
2) Batch upscale (4x) with ~15 different models (RGB/alpha separately, color-matched to the original)
3) The best upscale is chosen for each texture individually where the results are decent. In many cases, different upscales are blended, edited or redone from downscaled samples; some textures reupscaled by parts using different models.
4) Normals are either upscaled (RG channels, then unit vector normalized with the B channel) or regenerated from the upscaled diffuse maps.
5) Recompressed to dds with NVIDIA Texture Exporter.

Comparison screenshots
IC Aristocratic
IC Aristocratic 2
IC Interior Tileset
Aqueduct
Aqueduct 2
Wooden Sign
Baths Statue
Cauldron
Golden Horse Courier Board
Sofa
Bruma Castle
Apron (also fixed normal)
Lowres Imperial Legion sign
EMPIRE
Some furniture
Chorrol Cathedral
Leyawiin Armor
Fachwerk
True connoisseurs of the B&M Bravil retexture also not to be forgotten (others, don't worry, if you haven't installed B&M texture option, you won't have this): 1 2

Tools Used
Adobe Photoshop
chaiNNer
Topaz Gigapixel AI
NVIDIA Texture Exporter
Public models from OpenModelDB, thanks to Helaman, Crustaceous D, Kim2091, Venomalia, Mutin Choler, xinntao, CF2lter, RunDevelopment, Zheng Chen, mkultra, buildist
IrfanView
NVIDIA ICAT
Rudimentary Python automation

Credits
BC Team - for BC
Vorians - for maintaining the mod and steadily bringing it up to modern standards
ChaiNNer, Kart's and Enhance Everything Discords - for guidance
Gordon Ramsey's Kitchen Nightmares - was playing on one of my aux monitors to keep me sane during texture sorting