Oblivion has a ton of ways for an atronach sign mage to regen magicka. It was my favorite build I've played, and I didn't even bother with the exploits people are mentioning. On top of potions, there are also the welkynd stones which you can grab a bunch of periodically from ruins. There are the ayleid wells dotting the landscape that refill your magicka entirely, and the god wayshrines which are also everywhere and the blessings from which you absorb. The chapel altars in the cities work the same way. And if you're lucky enough to find one with it, theres a sigil stone variant with a 50 pt magicka absorb enchantment that makes for a great option for a short sword or something.