On the software side, you could install the Streamline mod. It has a feature (part of its "Streamsmooth" component) that can temporarily lower graphical settings (causing higher FPS) when you are in combat. It also has other features that can improve FPS too.
[An] optional smoothing feature is the FPS combat booster. If you want slightly higher FPS during combat, you can simply set the boost variable in the INI file a bit higher, and the smoother will keep FPS nearer to the top of your regular FPS range automatically. Say you set your FPS range to be 20-30, and the combat boost to be 0.8. During combat, the smoother will attempt to get FPS 80% of the way from 20 to 30 (right around 28). When combat ends, it will go back to normal adjustments. If you set it above 1.0 (which is 100%), you can effectively boost FPS during combat -over- your normal range.
From: Streamline's 'SL Tome of Knowledge.txt' file
I'd also recommend installing Oblivion Stutter Remover (OSR). It should also significantly help with your framerate. Note that OSR has an FPS cap, by default. If you're using OSR, make sure you set
MaximumFPS = 0 in
sr_Oblivion_Stutter_Remover.ini to avoid OSR putting an artificial cap in your framerate (this is set to 30, by default). I recommend setting
The vanilla Oblivion heap is unoptimized. OSR can replace the vanilla Oblivion heap with a different heap to improve performance. Set
bReplaceHeap = 1 and change
1 usually works fast enough. Read sr_Oblivion_Stutter_Remover.txt for more info.