Summon Frost Atronach is indeed still in the game, and it was the one I used the most, definitely (until I reached skill 100 in conjuration and could summon the über Lich). The Frost Atronach can heal itself, and is very powerful. The summon spells you can buy last fairly shortly, so I made my own spells with summon for 60 secs. (Wait until you see (and hear) the Storm Atronach in Oblivion, btw. Awesome!

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Btw. you should complete the Mages guild initiation quests as soon as possible, so that you can create your own spells at the Arcane University. As soon as you learn a paralyse spell from someone, create your own destruction spell with 1 second paralysation mixed in with some shock, fire and frost damage, and maybe also weakness to those (and weakness to magicka). Paralysation is a bit more realistic in Oblivion compared to Morrowind: the targets don't 'freeze', but instead fall to the ground. So 1 sec paralyse demobilises the targets for 3-5 seconds while they fall down and get back up. Very handy.
And yes, I think destruction spells are much better (as are all spells) in Oblivion compared to Morrowind, since it doesn't take forever from you initiate the spell and until the spell is actually cast. It happens immediately (like with scrolls from Morrowind). And the effects when you cast spells are awesome in Oblivion, I think. When you cast a shock spell, for example, you can see the lightning flashing towards the enemy, and you hear the sound of thunder/electricity, along with nice light effects. Visuals are very much improved in general.
Also remember the shortcut keys. You can assign keys 1-8 to weapons, spells, items etc. Very handy for not having to 'break the realism' by going into the menu system to find spells and weapons several times during a fight. Additionally the menu system is a bit better once you get used to it, imho. Spells are categorised on several pages (ranged, touch and self), so you don't have them all listed on one page.
Anyway, definitely give the command spells a spin, and may the force be with you...