So here's the deal. Oblivion stops responding as I enter Arena Bloodworks after defeating a khadjit warrior for a bloodletter rank. It works fine if I start oblivion without OBSE though. So how do I learn which of the mods causes this and disable it?

Obviously it's caused by 1 of your mods that require OBSE. The easiest method is to just disable half your mods and test. Then depending on the outcome enable or disable mods until you find the 1 causing the issue.

The first thing to do would be to post your load order from Wrye Bash (Go to Mods tab, Right Click File---> List Mods, Right Click---> Select All, Copy and Paste here) so we can see any obvious candidates (Out of date or obviously conflicting mods)

I would advise you to Run BOSS(to make sure your load order is correct) and take a careful read of the BOSS Log. Pay attention to any warnings about conflicting mods, or dirty esps (follow the link given on there to learn how to clean mods if you don't already know how)

If nothing obvious arises from those steps you will have to resort to the "binary method" in order to track down the conflict:

First deselect the Bashed Patch and load up your game, if your problem persists then the Bashed Patch wasn't the cause. If it goes away then something in the Bashed Patch is causing this (this is the least likely scenario by the way but it's still possible, and awkward if it is). You will have to effectively go through each stage of the Bashed Patch process one at a time in order to track this. In the rebuild screen uncheck everything except Merge Patches, select all of these and build the patch.

If the problem goes away after loading then one of the Mergeable esps is the cause of the problem. Rebuild your patch but this time deselect half of your merge plugins; if the problem goes away rebuild again and deselect half of your plugins again (so you are left with a quarter ticked); if the problem doesn't go away, reselect half of your remaining plugins (so three quarters are now selected) and rebuild your patch. Repeat this process, taking the necessary action depending on the result until you discover which of your mergeable plugins is causing the issue; depending on how many plugins you have this will probably take 3-5 passes before you can confidently name the offending plugin. You can then temporarily remove that plugin and continue on your way.

If none of your Merge Patches were responsible for the issue then move on to Import Actors and follow the same process as before. Work your way down the Import selections until you locate the problem.

Now if the Bashed Patch wasn't an issue then you can use a similar method for your other mods. Starting from the bottom (you don't want to be deactivating your esms!) deactivate half your plugins and load the game. If the problem goes away, one of your plugins in the bottom half is your problem. Reactivate half of what is left (so three quarters are now active) and load the game again. If the problem persists your issue is with the top end, deselect half of you plugins again (so you are left with a quarter active) and load your game. Repeat and take appropriate action until you find the offending plugin.

Hope my rambling answer helps you, I've tried to be as concise as possible.

Artorius.

Only problem with that, if it's an OBSE Plugin. (Streamline, OSR, Pluggy...).., it wouldn't be listed...