Hi There!

So there I was, starting a new Oblivion game after almost three years without playing a game (full full full time job) and all of a sudden an Imperial guard walks by without a torso! I guess that means there's a missing texture somewhere. How do I fix it?

(I have a few mods that might have overwritten the textures, such as OOO, but I'm not a modder and don't really know how that works.)

Yeah I get that and the occasional boxed ! which is annoying

If you have a body replacer, make sure you are using a clothing and armor replacer that is designed for the body replacer. Many of these issues can be avoided by paying attention to the order you install your mods, and using Bsa Redirection for Archive Invalidation.
To avoid invisible objects a mesh needs to have the correct textures, and an esp needs to have the mesh properly installed in the Meshes folder of Oblivion\Data for the item to show up in the game.
Use the first link in my signature to find out more about how this works, and if you post a Load Order, I might be able to tell you what mod you need to fix the problems.

http://http://www.tesnexus.com/downloads/file.php?id=16704 may be a tool you'll find useful.
Also check the known issues with version 1.33 in the opening post of the OOO Homepage.

As bradleyh pointed out, make sure you're running BSA Redirection via Wrye Bash or OBMM. Another user had exactly this issue just yesterday - guess because those particular cuirasses are mod added but also are using vanilla resources. Anyway, BSA Redirection is basically a must.