One of my funnest Skyrim playthroughs was an illusion/conjuration assassin that had very few direct kills - I'd kill off most of my enemies with frenzy, and let atronachs loose to finish the rest. I wonder if this would be a viable build in Oblivion. My biggest concern is with illusion. It's been a long time since I played the game at all, so I can't remember the details, but isn't the max level for frenzy/fear/pacify around 30? Would this stop my character from being able to function at high levels? Am I wrong, or is there a workaround or something?

Max level of many illusion spell effects is 25, but they continue to work on any enemy, at any level, as long as your spellcasting is at 100%. This simply means that you cannot wear any armor, in order to get the needed efficiency.

That could work, thanks. Do you know of any means of raising this or working around it, like illusion buffs or anything?

There's no need to raise it (and no, you can't, as it's the fixed upper limit at the spellmaking altar.) Just don't wear armor, and the 25 value will work fine on anybody at any level. (You can enchant shield effects onto regular clothing if you want, and get an armor rating without an armor penalty.)

Cool. Thanks again!

My mystic archer's magic specialty is illusion. Normally she snipes isolated targets. When she encounters crowds, she uses command spells to get them to fight for her and thin the crowd down to one wounded survivor - then she snipes him. Her quiver has a limited capacity and when she sometimes runs out of arrows, she substitutes a summon for her bow to do the clean up.

A pure illusion followed by summon could work. It actually works better vs crowds where your summon only has to clean up. It is a little bit harder vs big solo foes where your summon would have to do all the work, but there is a technique to help there as well - buff up your summon with spells as you send them into the fight.

Using illusion is great fun. Sometimes you can even command the summons of your foes to turn on their masters. If you intend to take your character to higher levels, then you do indeed want 100% spell effectiveness which ensures that max magnitude mind control spells will work vs any foe.

I have pretty much that exact character in Oblivion, though he uses command much more than frenzy (since it protects him). He's a dissolute alcoholic assassin from a noble family fallen on hard times who's too proud to do anything that smacks of the sort of "work" that "peasants" do, so he refuses to touch a weapon, or to use any direct magic. He uses Illusion and Conjuration pretty much exclusively, and to great success. He doesn't do anything as seedy as going into dungeons, so the only random enemies he has to deal with are occasional roadside creatures and bandits and such, and they often conveniently come in pairs. The only real problem, as Acadian notes, is when they're solo. Depending on the enemy and the situation, he's most likely then to just use Demoralize and keep going. He only hangs around if the enemy has some loot he really wants, and then he's most likely to just keep summoning stuff until the opponent's dead. Depending on the opponent, that can be either hard or easy. I recognize the practicality of buffing a summon, but he doesn't. He just rages at them when they fail.

As noted, the upper end is actually level 25, but that functions against all levels IF you're at 100% spell effectiveness, so it's of no note unless you want to wear armor (which is really sort of pointless anyway, since enchanted clothing offers at least the same if not better protection, weighs less and doesn't have to be repaired).

Oh, a couple practical spell-crafting tips my character, Buffy, uses that help keep her spellbook simpler is that she combines effects to keep things simpler and safer - Examples:

Command Humanoid + Command Creature + Invisiblility on self . This spell makes sure her 'Command' spell works on any foe in the game and she is safe for a few seconds after casting it.

Demoralize + Turn Undead + Invisibility on self. Same idea here - a universal 'Fear' spell. This one was came to be after Buffy (in a panic) accidentally cast a demoralize spell on a wraith - the spectre ignored the spell nearly killed Buffy. So her thinking often comes from hard experience. One cautionary and odd note here is that demoralize is not considered hostile while turn undead is. The point being that when she wants to simply tease nasty and mean NPCs (Alval Uvani, I'm lookin' at you), she is careful to use only a pure demoralize spell so she doesn't have any troubs with her friends in the Legion or city guard forces. The point is, well, pointless in actual combat however.

Summon a helper + invisibility on self. This pops a meat shield in front of her and ensures (because she disappears in the process) that the summon instantly gains the full attention of her foes. Once her foe 'locks on' to the summon, then she can reappear behind the summon to cast a combo buffing spell (or heal the summon) if needed. The buffing can pretty effectively help a clannfear tromp a storm atronach.