Step one: Enter Desktop mode (Click the steam button in the lower left, Power settings, Enter Desktop Mode

Step two: Open the Konsole (Press the button on the lower left of the taskbar, type into the Search bar at the top of the menu that opens "Konsole" or "Terminal", double tap the icon that shows up)
Step three: Copy the following script:

bash <(curl -s https://raw.githubusercontent.com/krupar101/sd_oblivion_remaster_scripts/refs/heads/main/optimize_oblivion_remastered_for_steam_deck.sh)

Past the script into your Konsole (Use right click instead of Ctrl C and Ctrl V, sometimes extra stuff gets pasted into Konsole using these)
Step four: Press enter and allow the script to run and install your optimizations. Run the game in Gaming Mode (You can press the button on the lower right of the taskbar to show your desktop then click the "Return to Gaming Mode" icon to return if you're unfamiliar with Desktop mode) to ensure full performance. DO NOT TWEAK YOUR SETTINGS, it may be tempting to try and bump one of them up from where this script puts them but trust me, its worth it for the frame rates.

Credit goes to https://overkill.wtf/steam-deck-oblivion-remaster-automatic-optimization/ for this method, which has been the only method I've been able to find that makes the game not run between 20-30, but actually goes 30+ with very few dips.