Air Carrier First Officer reported a fumes event prior to pushback. Maintenance was requested and passengers were deplaned.

Narrative

Prior to a delayed pushback due to inclement weather; the Captain and I started smelling oily and musty fumes. We received a call from one of our flight attendants almost simultaneously and she commented that the smell was quite thick in the cabin and customers were complaining and covering their noses. The bleed and packs were shut off and operations were called to reattach the jet bridge. After coordinating with the agents; the aircraft was deplaned; paramedics requested for the flight attendants and Dispatch and Maintenance were contacted. Maintenance informed us that when the APU seal breaks on the Honeywell-131 APU; it does so without warning. He went on to inform us that the moisture from the inclement weather exacerbated the fume event. Furthermore; he remarked that the seal failure was one of the worst he had seen. He was able to duplicate the event almost immediately upon inspection. I believe there may be an issue or consideration of the APU type utilized in the aircraft. It's design may contribute to these types of failures. If that's the case; the manufacturer is likely aware and may offer additional insight.