Air carrier Captain reported an oil leak or spill in the cargo bin during preflight. Ground personnel along with the Fire Department cleared the spill and the flight departed.

Narrative

ZZZ1-ZZZ flight ABC into ZZZ. Had an incident while performing post flight walkaround where I was almost hit by luggage from above coming out of FWD cargo bin. As I completed my walk around and was about to head back up the jetway stairs; the Ramp Agent who was in the cargo bin approached me to apologize. He stated the reason the bags were coming off the aircraft like that is because there is an oil spill or leak in the cargo bin causing the bin floor to be extremely "slippery" I asked him to confirm it was oil; to which he replied "Yes". I went up the stairs to discuss the issue with the F/O. We agreed it needed to be written up and the baggage loading needed to stop for the time being. I informed Supervisor to inform Ramp Agent to not load bags (based on the fact that they had just got done unloading I assumed they had not started loading yet). Flightcrew further discussed if this was in fact a hazmat situation and determined that unknown oil very well may be. I seemed to remember a need to notify the airport authority of possible hazmat situation from our training program.I had Supervisor connect me with the local airport authority and they informed me they would send the fire dept over to inspect. There was a rather unproductive conversation with Maintenance Control on site as to whether or not to consider it may be hazmat and also whether or not the Airport authority had to be called. It was moot at this point as I had already written it up in the logbook and notified the Airport. Fire Dept cleared the aircraft; bag loading resumed; and logbook signed off. Flight departed; safely; and without concern there was a dangerous; unknown "oil" substance onboard.As per FOM; Dispatch was also notified of the possibility of hazmat on the aircraft. They informed the On Duty Supervisor. They subsequently called me for more details; which I gave him. He recommended a report in addition the other report."