Autocracy: 1) dictator, or monarch is the sole ruler 2) Ruler is
reliant on a power structure of loyal nobles
Oligarchy: 1) Is "rule by the few" 2) Oligarchies have been tyrannical throughout history, being completely reliant on public servitude to exist.
The autocrat needs some kind of power structure to rule. Very few rulers were in the position to rule with only their personal charisma and skills, however great these may be, without the help of others. Most historical autocrats depended on their nobles, the military, the priesthood or others, who could turn against the ruler and depose or murder them. As such, it can be difficult to draw a clear line between historical autocracies and oligarchies. http://en.wikipedia.org/wiki/Autocracy
A form of power structure in which power effectively rests with a small segment of society distinguished by royalty, wealth, family ties, or military control. http://en.wikipedia.org/wiki/Oligarchy