Air carrier Flight Attendant reported a fumes event during preflight prior to boarding. The aircraft was removed from service.

Narrative

We arrived to the gate 25 minutes prior to boarding. waiting on the gate agent was not present yet. 15 minutes prior to boarding the gate agent arrived. FA3 and myself FA4 scanned our badges and went down to the aircraft. The flight had arrived approximately 30 minutes prior. About half way down the jet bridge myself and FA3 began to smell an extremely strong musty odor; I identified as oil. We proceeded to the aircraft where the smell was extremely strong; sweet musty odor. The aircraft was being catered; the caterer said maintenance was working on the tail and that's why it smelled. They were referring the APU.I immediately turned around and evacuated myself off the aircraft to the gate counter. I informed the gate agent and pilot regarding the fume event and we should not begin boarding. The plane was taken out of service. Another aircraft had a prior write up for ECAM Low APU Oil Quantity the night before and the same wrote up that morning. Logbook data shows the aircraft was confirmed to be leaking oil into the APU intake. Crew members FA1 and FA2 were on the aircraft for an extended amount of time and reported feeling ill.