I've read a few threads and wiki articles about the leveling problem in Oblivion. Found lots of good info at the UESP wiki. I thought I read about a specific quest where this issue makes it particularly hard, where you have an NPC helper and the leveled monster might be too hard for him. I haven't had much trouble so far.
I'm a lvl 20 Battlemage and have focused on Str and End, and supplement with potions and summoned critters. My next main quest is the Miscarcand where I've read there is a tough lich. I just found a mace with fire damage which has made undead encounters much easier. But I've recently 'wasted' a couple levels leveling up Alteration so I can get to 75 to get unlock Hard locks.