After an A320's APU was started at the gate and APU bleed air selected for air conditioning the cabin and aft cargo pit filled with the dirty socks odor. Oil from an over serviced APU oil reservoir may be a cause.

Narrative

Upon arrival at gate; and after the last passenger deplaned; I engaged the APU air for cabin comfort (we were running late and elected to use the APU to facilitate a quick turn for the subsequent flight). Within a minute or two; I and the 'A' Flight Attendant noticed a strong 'dirty sock' odor. I called for the 'B' Flight Attendant to come forward and instructed all crew members to leave the aircraft. Maintenance personnel arrived as we were leaving the aircraft; so I requested their opinion about the odor and they quickly concurred with my suspicions that the APU bleed air was causing the odor. At about this time a Ramp Agent came up to the jet way; where we were all gathered; and remarked that he had noticed a very strong 'dirty sock smell in the cargo compartment.' I entered the event in the logbook and completed a 'cabin air' event form. After waiting for the cabin to be clear of any detectable odor (we had turned off the APU air); we retrieved our belongings and prepared for rerouting or hotel instructions. After some time the mechanics took the aircraft to the hangar. To reiterate; the only people on board the aircraft at the very first detection of the odor were the 5 crew members; and we all left the aircraft within 2 minutes of noticing it. No one reported any symptoms that might be associated with aero-toxic syndrome.