EDIT: The vanilla elemental shield potions don't work either. Checking with TES4Edit it's only OOO and Supreme Magicka that affect the elemental shield effects, but that's only durations etc. Also, Fortify X potions work - the issue only seems to affect shield potions.
EDIT: Casting a Frost Shield spell doesn't up the AR either, so the issue seems to be with elemental shield effects not upping AR at all.
The main thing I've done recently is upgrading Wrye Bash from 290 to 291 and rebuilding the Bashed Patch. I've also updated Supreme Magicka to 0.90b, but I think I should have noticed the issue since as my char relies a lot on potion buffs - not 100% sure about that.
Anyway, I have really no idea where to even start to look for what might affect this. Does anyone have an idea? Here's my current load order:
Spoiler
Active Mod Files:00 Oblivion.esm01 All Natural Base.esm [Version 1.2.1]02 Cobl Main.esm [Version 1.73]03 Oscuro's_Oblivion_Overhaul.esm [Version 1.35Alpha2]04 Mart's Monster Mod.esm [Version 3.7b7]05 FCOM_Convergence.esm [Version 0.9.9MB6]06 Armamentarium.esm [Version 1.35]07 Artifacts.esm [Version 1.1]08 Progress.esm [Version 2.2]09 CustomSpellIcons.esm0A Unofficial Oblivion Patch.esp [Version 3.2.8]0B DLCShiveringIsles.esp0C Unofficial Shivering Isles Patch.esp [Version 1.4.0]0D All Natural.esp [Version 1.2]0E All Natural - SI.esp [Version 1.2.1]0F All Natural - Varla - Welkynd - Real Lights.esp10 Immersive Interiors.esp [Version 0.7]11 Immersive Interiors - Lights Addon.esp [Version 0.7]12 Enhanced Water v2.0 HDMI.esp++ Symphony of Violence.esp13 WindowLightingSystem.esp** Book Jackets Oblivion.esp++ Cobl Bookjackets.esp14 FlyingCameraMod.esp [Version 1.3]15 VanityCameraSmoother.esp16 DS Time Manager.esp [Version 1.1]17 ImprovedSigns.esp18 FormID Finder4.esp19 P1DmenuEscape.esp1A Streamline 3.1.esp1B RAEVWD Cities.esp [Version 1.9]1C DLCOrrery.esp1D DLCOrrery - Unofficial Patch.esp [Version 1.0.4]1E DLCVileLair.esp1F DLCVileLair - Unofficial Patch.esp [Version 1.0.6]20 DLCMehrunesRazor.esp21 DLCMehrunesRazor - Unofficial Patch.esp [Version 1.0.5]22 DLCSpellTomes.esp++ DLCSpellTomes - Unofficial Patch.esp [Version 1.0.1]** Book Jackets DLC.esp** GrimbotsSpellTomes.esp++ MaleBodyReplacerV4.esp23 Adonnays Classical Weaponry.esp24 Adonnays Elven Weaponry.esp25 Adonnays Elven Weaponry for NPCs.esp++ Adonnays Elven Weaponry for NPCs - Armamentarium piggyback.esp26 Binoculars.esp27 Blackmarsh_Witch_HGEC_Clothing.esp28 Captain_Molly_HGEC.esp29 DG Jewelry for All.esp2A DM-Lingerie.esp2B Forestlight Armor.esp2C KDClothCirclets.esp2D KDDwemerSpectacles.esp2E kikaiAS-SheogorathRT-sep.esp2F PTArtifacts.esp30 PTAutumnTrees.esp31 PTFallingStars.esp32 Rogue Sun's Blood Lust Dress.esp33 Sinblood's Widow Dress.esp34 Sinbloods_Nightweave_HGEC.esp35 1SB_RavenousEleganceOutfit.esp36 trfj_merged.esp37 HGEC_SBBewitching.esp38 ShadowWatcher_HGEC_v2.esp39 Armamentium female.esp3A R18PN - Dancer's Dress.esp3B Weapon Expansion Pack for Oblivion Nthusiasts.esp [Version 1.03]** Slof's Oblivion Robe Trader.esp3C Cobl Glue.esp [Version 1.73]3D Cobl Si.esp [Version 1.63]3E Oblivion WarCry EV.esp [Version 1.09]3F FCOM_WarCry.esp [Version 0.9.9MB5]40 Oscuro's_Oblivion_Overhaul.esp [Version 1.35Alpha2]** OMOBS.esp [Version 1.1]** OMOBS_SI.esp [Version 1.1]41 ArmamentariumArtifacts.esp [Version 1.35]** OOO 1.32-Cobl.esp [Version 1.72]42 FCOM_Convergence.esp [Version 0.9.9Mb6]++ FCOM_EntropicOrderRebalance.esp [Version 0.9.9beta2]43 FCOM_RealSwords.esp [Version 0.9.9]44 FCOM_DurabilityAndDamage.esp [Version 0.9.9]++ FCOM_SpawnRatesStronger.esp [Version 0.9.9Mb4]45 Mart's Monster Mod - More Wilderness Life.esp [Version 3.7b7]++ FCOM_LessRats.esp [Version 0.9.9]++ FCOM_NoWyverns.esp [Version 0.9.9]46 Mart's Monster Mod - Extra Wounding.esp [Version 3.7b7]++ Mart's Monster Mod - No Reavers.esp [Version 3.7b7]47 Mart's Monster Mod - No Carrion Rats.esp [Version 3.7b4]++ Mart's Monster Mod - No Bone Loot.esp [Version 3.7b7]++ Mart's Monster Mod - No Slimes.esp [Version 3.7b7]++ Mart's Monster Mod - No Fiends.esp [Version 3.7b7]++ Mart's Monster Mod - No Lanterns.esp [Version 3.7b7]++ Mart's Monster Mod - No Beholdens.esp [Version 3.7b7]++ Mart's Monster Mod - No Uberhulks.esp [Version 3.7b7]48 Mart's Monster Mod - Zombies for Body Meshes.esp [Version 3.7b7]49 Mart's Monster Mod - Safer Quest Locations.esp [Version 3.7b7]++ Mart's Monster Mod - Diverse Runeskulls Loot OOO.esp [Version 3.7b7]++ FCOM_DiverseGuardUnity.esp [Version 0.9.9]++ Mart's Monster Mod - Shivering Isles.esp [Version 3.7b7]++ ArmamentariumLL4OOO.esp [Version 2.01]++ ArmamentariumLLMagicOOO.esp [Version 1.35]++ MMM-Cobl.esp [Version 1.73]** EVE_StockEquipmentReplacer4FCOM.esp4A Plate Armor Plus - OOO_FCOM_version.esp4B Ivellon.esp [Version 1.8]4C ZUAmberMadnessOverhaul.esp++ ZUSIO-MMM SI-patch.esp** ArtifactsFemaleArmor.esp++ Artifacts - ArmaCompleteAddon.esp [Version 1.1]4D BenirusManorRebuilt.esp4E Gift of Kynareth.esp4F TheForgottenShields.esp50 DLCFrostcrag.esp51 DLCFrostcrag - Unofficial Patch.esp [Version 1.0.5]52 FrostcragAlchemySorter.esp53 Knights.esp** Knights - Book Jackets.esp54 Knights - Unofficial Patch.esp [Version 1.1]** EVE_KnightsoftheNine.esp55 Mighty Umbra.esp++ FCOM_MightyUmbra.esp [Version 0.9.9]++ FCOM_Knights.esp [Version 0.9.9Mb6]56 Gah Falquan Umarile.esp57 Bag of Holding.esp [Version 1.5.0]58 EVE_ShiveringIslesEasterEggs.esp59 Respawning Varla & Welkynd Stones.esp5A Teleport Everywhere.esp5B Toggleable Quantity Prompt.esp [Version 3.2.0]5C ATakesAll.esp5D Quest Award Leveller.esp [Version 2.0.1]++ Quest Award Leveller - Vile Lair.esp++ Quest Award Leveller - Mehrunes Razor.esp [Version 2.1.0]++ Quest Award Leveller - Knights of the Nine.esp [Version 2.1.0]5E RealisticFatigue.esp5F StarX Vampire Deaths.esp60 SupremeMagicka.esp [Version 0.90b]61 SM_ShiveringIsles.esp [Version 0.90]++ SM_DLCSpellTome.esp [Version 0.90]++ SM_OOO.esp [Version 0.90]++ SM_MMM.esp [Version 0.90]++ SM_COBL.esp [Version 0.90]++ SM_Scrolls.esp [Version 0.90]++ SM_SigilStone.esp [Version 0.90]62 MidasSpells.esp++ Midas Assimulator Addon - Temp Fix.esp63 Midas Assimulator Addon - Oscuro.esp++ Mart's Monster Mod - Midas Creature Ingredients.esp64 SigilStoneSelector.esp65 Syc_AtHomeAlchemy_No_Bounty_v2.esp66 EnchantmentRestore.esp67 Enhanced Hotkeys.esp [Version 2.1.1]68 ProgressMBSP.esp [Version 2.0]69 ProgressSBSP.esp [Version 1.0]6A RealisticLeveling.esp6B Companion Compatibility.esp++ EVE_KhajiitFix.esp6C Better Redguard v2.esp6D Moonshadow Elves - NoSc.esp6E 1em_Vilja.esp [Version 3.1]6F AleswellHomeQuest.esp70 ViljainAleswell.esp71 CompanionMaster.esp72 Cobl Tweaks - FCOM.esp [Version 1.72]73 Cobl Races.esp [Version 1.52]** Cobl Races TNR.esp [Version 1.53]** Cobl Races TNR SI.esp [Version 1.53]74 MRP - Loading area.esp75 [GFX]_Initial_Glow-all.esp++ bgMagicShaderLifeDetect.esp [Version 1.68]76 Shining Creatures.esp++ Shining Creatures_DLCFrostcrag.esp++ xs_WarCry.esp++ xs_MMM.esp77 xs_OOO.esp++ xs_armamentarium.esp78 xs_Rogue Hair.esp++ xs_si_playable.esp79 Mart's Monster Mod - Diverse Creatures Expansion.esp [Version 3.7b6a]++ MMM Balrog No Fleeing.esp++ MMM More Spriggan and Ent Rain.esp++ OOO Harvest Containers Filter patch for mods.esp [Version 1.03]** All Natural - Indoor Weather Filter For Mods.esp [Version 1.2]7A Bashed Patch, 0.esp
The xs ones are my own tweaks and shouldn't have anything to do with this.