Oblivion:Aggression

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Aggression determines the Disposition level at which an NPC will attack. If the NPC's disposition towards you or any other NPC drops more than five points below the NPC's aggression, the NPC will attack. NPCs with an aggression of 5 or less will never attack, and NPCs with an aggression of 0 will never fight, even in self-defense. NPCs with an aggression of 6 or more will attack you if their disposition is lowered enough.

Aggression and Disposition determine combat for the game. Creatures and hostile NPCs such as wolves and bandits have high aggression and automatic low disposition, causing them to attack. Other NPCs and creatures that may attack, such as highwaymen (will not attack if paid) and Mythic Dawn Assassins (will not attack until later on in the Main Quest line) will have disposition and aggression changed to determine their likelihood to attack.

Since NPCs decide whether or not to accept a yield based only on disposition, changing aggression is a common tactic to have the NPC attack without refusing a yield. On the other hand, disposition is used where aggression could become unstable or would not make sense, e.g. a Guard with a Disposition of 80 toward you would attack the whole town if their aggression was raised that high.