When you uninstalled, did you keep save-files?
You'll probably need to delete the folder from My Docs as well (...\My Documents\My Games\Oblivion - on Windows anyway)
I'd also delete the WryeBash / OBMM folders along with everything else.

I also had trouble with certain mods (or at least the install order).
Using WryeBash helps but if you get the install order wrong, it can still cause crashes.

Bear in mind that some mod-folders don't use the standard structure, and some mods conflict with each other.

Once you've got Oblivion vanilla loading up without crashing, start again adding one mod at a time (using WryeBash so you can uninstall / deactivate the offending mod more easily).

(Note that for texture replacement mods, I had an issue even with WryeBash that uninstalling one left me with purple-billboard grass).
I'm planning on a full reinstall myself.

[url=http://wiki.theassimilationlab.com/tescosi/A_General_Order_for_Installing_Mods_(Oblivion]http://wiki.theassimilationlab.com/tescosi/A_General_Order_for_Installing_Mods_(Oblivion[/url])
^This might help with install order issues

As for square screen v. widescreen - what options do you see under 'video' in the options menu? I've got 1680x1050 (my native res) and up to 1440x900. (Not sure if OBSE is enabling that or whether it's native). Set Texture Size to 'Large' too - there are additional options but you need to go into the config file (oblivion.ini - there's one in the main oblivion folder and one in 'My Games\Oblivion' (main one is the default, MyGames is the one for the current user. Wrye Bash can help edit this file too but be careful with editing it.

Got it! I deleted the save files, AND I turned off cloud syncing, and now I can start the game again.

Now for the mods... here's hoping it works better than last time.

Cool - good luck :)
(and see my edit above for higher res widescreen Q)

Don't forget the unnofficial patches (Oblivion, Oblivion DLC, and Shivering Isle versions, in that order).

Here's how you should do it:
First install OBSE because a mod manager can only install files to your data folder "Oblivion\Data" and not to your game folder "Oblivion".
Then install a mod manager and use it to install one mod at a time!
That way you won't have to un-install your game if something goes wrong.
When you've installed a mod, test if your game runs. Say, it simply crashes like before… There's usually a simple explanation for that: You put a mod that requires another mod before the mod it requires. (Or, of course, you don't have the required mod to begin with.) You can check that by selecting the mod files in your mod manager with your mouse. The mod manager tells you what any mod requires.

Lastly, you should know what belongs into the data folder: mod files like ".esp", ".esm" and mod folders like "Meshes", "Textures". Your mods normally come in archives that you need to extract (using 7zip for example). A mod manager handles archives for you, but you usually have to extract the mod archives anyway and make new ones. That's because the files like ".esp" and "Textures" have to be at the very top level in your archive.
It's very simple: The mod manager installs the contents of your mod archive ("Mod.zip") to your Data folder, but that can be an issue as I'm going to show now. The contents of the archive might for example be: (Mod.zip\)Mod\Data Files\Mod.esp
In this case your Oblivion directory would look like this: Oblivion\Data\Mod\Data\Mod.esp. Your game won't realize that there even is an .esp. That's why you make a new archive that contains Mod.esp on its top level and feed the new archive to your mod manager. (Mod.zip\)Mod.esp would lead to a correctly installed mod Oblivion\Data\Mod.esp

Hope this helps.

Edit: oops, the name of the folder is "data", "data files" is a Morrowind folder.

The latest version of Oblivion Reloaded crashes on "New game" (even if you just install that and don't install anything else). Download the last 6.1.x version (I use 6.1.2) which works perfectly so far. However, in general if you are new to Oblivion modding you need to start with something much lighter. Oblivion Reloaded does a lot of changes.

Shameless plug: I recommend you use my guide on this matter, it is written from the perspective of someone that hasn't modded Oblivion before. It will walk you through various mods from non-risky texture/sound updates to heavily scripted/gameplay changing mods. At various stages in the guide there are test breaks where you need to check that things still seem to be working well. All mods have been and are being tested by me playing many hours with them installed. If you run into issues or have questions please let me know (add Galaxy chat, comments on the medium page or forum private messages).

I'm so glad I'm not the only idiot to have done this... Thanks! Got it going again!