Sorry, but this is incorrect. In Oblivion's combat, you ALWAYS do the same amount of damage with a given weapon at a particular level of skill and Strength, unless it's reduced for low Fatigue. There is no D20 roll, or any other random element to damage, in OB, or in Morrowind (which varied weapon damage linearly between minimum and maximum for the weapon according to how long you "charged" the attack before releasing, NOT randomly).
Yup, and the weapons even seem to have a hidden accuracy value that works similar to that power value (the longer you "charge", the harder the weapon will hit in its X - Y scale). At least for me it seems that the longer I "charge" the attack (to a certain point; I guess the maximum is when the weapon is fully drawn back and not moving anymore), not only does it hit harder, but more likely as well. Is there truth to this? I remember this being discussed before but I don't think I've seen a clarifying answer to this.
So, is it so that the most powerful attack is also the most accurate one? In my experience, it really seems to be. Not many drawbacks in that case to always use the "fully-charged" attack, especially if there's a big difference in the weapon's minimum and maximum attack power and your skill in that weapon is pretty low. It takes "X->0"-to-2 seconds longer than less charged attacks, that's it about its drawbacks.