Tips for pure mage build

[Dexter]

Returned to Oblivion after so long and I never ever done pure mage playthrough. Are there any tips you can share or hidden mechanics(like each piece of armor giving decreased magicka) to keep in mind?

[WingedKagouti]

Armor doesn't inherently decrease your Magicka, but it does decrease the efficiency of all spells you cast. If you're wearing any armor (including shields), your spells will never be as powerful as if you're wearing (non-armor) clothes.

The Magicka cost to cast a spell decreases as you increase your skill. So you don't actually need 400+ Magicka to cast those big spells sold by various members of the Mages' Guild.

Once you get access to Spellmaking and Enchanting, whether by getting access to the Arcane University or via plugins (or mods), you should note that you can only use effects from spells (and Powers) you're actually able to cast. If the only Frost Damage spell you know requires Expert Destruction and you're only at Apprentice, you won't be able to add Frost Damage to any new spells or items.

Don't pick Alchemy as a Major skill, even if you want to do a Pure Mage build. As a Major skill, you really can't level it early on as it not only very easy to level up but it also doesn't help you as directly in combat as the various spell schools. But it's a very useful skill for a Mage, since you can make reasonably light (0.1 weight) Healing and Magicka potions that will be as effective as most potions you can buy but at a lower cost. One thing to keep in mind, Alchemy skill level (base skill, Fortify Skill and Luck bonus) is effectively capped at 100, unlike Morrowind and Skyrim you can't boost various stats to make better potions than your current Alchemy gear lets you do at skill 100.

While any race can be a Mage, if you're going for the Pure Mage experience Breton and Altmer are the two most iconic options in part due to their extra Magicka (but also lore reasons). While Bretons do have Magicka resistance, both genders also start with lower Endurance than Male Altmer which can be dangerous in early game combat. But Altmer have their elemental weakness, which is a liability when facing other mages or magical traps.

Of the Birthsigns, The Atronach needs careful consideration as the downside is as noticable as the upsides, unless you carry a stockpile of Restore Magicka potions around with you at all times. You can't really rely on the Spell Absorption to give you the Magicka you need for a vital spell. The Apprentice is near suicidal for Altmer, but Bretons can handle the penality. The Mage is decent if you don't want to deal with the other two. The Lady, The Thief and The Warrior aren't bad for a caster, but to me they don't feel like "Pure Mage".

[slayor3000]

Go for it. Mages built right are totally OP'ed, just remember to have a weapon as a backup.
Atronach birthsign is difficult, but when you get used to it and how to keep yourself "topped up", it is OP'ed. Welkynd stones early on and then alchemy.
If you play with Atronach Birthsign you can "almost" disregard Willpower, the only thing it is useful for is increasing fatigue.
If you have Frostcrag and grab the Apprentice stone from just outside Skingrad, that will give you +35 to alchemy. Also running around Skingrad to Imperial City you should be able to collect a lot of ingredients for "restore Magicka" potions.

So as stated above, Do Not put alchemy into Majors, it is to easy to raise and use. Keep it for raising int and selling potions.
Now this might sound strange but a Female Orc makes for a great mage or Battle Mage. They start with good Endurance and Strength, enough intelligence to make use of the Atronach Birth Sign (remember Willpower not that important) and have a 25% resistance to magicka. The fact they have no magic skills in there "portfolio" is unimportant, it only takes 1 or 2 levels to get them where they need to be.

[LUKE]

Leveling a pure mage in vanilla Oblivion can be a tedious and min-max-inducing process. To mitigate some of this I strongly suggest either of the following mods --> 'Easy Attributes' or 'All 5+ Attribute Modifiers' so you don't have to subject yourself to "Efficient Leveling".

Choosing The Mage stone + Breton is 100% optimal (avoid High Elf + The Atronach stone for your first pure caster playthrough).

Also, avoid choosing The Mage class or picking alteration, mysticism, illusion, restoration, alchemy, etc as major skills because you will level far too fast to ensure you have adequate combat skills to deal with the new monsters/enemies.

***Having too many utility schools as major skills will also make ensureing +5 int/will on level up virtually impossible without one of the mods I suggest above.***

Furthermore, consider choosing a few dummy major skills you do not intend to level to help with this issue. It might sound unorthodox, but it is optimal in this instance.

Random Tip: Daedra are resistant to fire and weak to lightning ;)

[Dexter]

Oh yeah! Forgot about stupid leveling system. Thanks everyone for replies

[WingedKagouti]

If you're actively looking to min/max and don't want to use a mod, you need to think a lot about your Major skills. You don't want ones you frequently use but you also want skills that are fairly easy to increase. Your primary weapon and armor skills should not be Major or you risk levelling up before you're ready. Acrobatics, Athletics, Sneak, and Speechcraft are also big no-nos.

As a general rule, I recommend either using a mod to fix it or not worry about securing +5s for anything but Endurance and Intelligence (until they're maxed).

[WingedKagouti]

Almost forgot two of the most important combat magic tips:

The Weakness to Magicka spell you can purchase is actually Weakness to Magic, that means all types of magic and not just non-elemental hostile spells. It's part of the reason why The Apprentice is so dangerous to Altmer. BUT, it's also one of the main ways a Mage can easily deal with most enemies, since it is able to affect future applications of Weakness to Magic (ie. it stacks with itself).

Consider the following spell: Weakness to Magic, Touch, Magnitude 100, 4 seconds. Cast it once (with 100% spell effectiveness) and a baseline enemy will now have 100% Weakness to Magic (ie. all future effects are twice as powerful). If you then cast it again before it wears off (the 4 seconds duration helps here), the enemy will now have 200% Weakness to Magic (3x strength). This means you can just layer the fairly cheap WtM until one of your (likely more expensive) spells can kill in a single cast.

The other tip: Drain Health is a deceptively powerful effect, while it merely decreases the current & maximum health of the target for the duration if the magnitude is higher than the target's remaining Health they instantly die. Drain Health, Touch, Magnitude 100, 1 second will be enough to kill most low level enemies in one cast. If you need to kill something with more Health, look at the Weakness to Magic loop.

[slayor3000]

To stack spells they need different names.
An example from the Uesp:
"One way of making super-powerful spells is by stacking spells. This is based on two facts:

Effects from spells with different names stack by adding together.
Weakness to Magic amplifies any following harmful spell, including weakness to magic itself.

To use spell stacking you need to start by making three custom spells.

Prepare 1:
Weakness to Fire 100% for 5 secs on Touch
Weakness to Magic 100% for 5 secs on Touch
Prepare 2:
Weakness to Fire 100% for 5 secs on Touch
Weakness to Magic 100% for 5 secs on Touch
Fatality:
Fire Damage 10 pts for 10 secs on Touch"
That combination would end up doing 1600 fire damage.

Beware of any high powered spells against Liches as they have a 25% "reflect", even with The Atronach and Resist Magicka, as a mage you usually have lowish Health and could possibly end up killing yourself.