If the cups in the master cylinder are leaking, brake fluid can travel along the push rod and enter the passenger compartment. The dust boot on the push rod usually keeps any fluid out, but I have seen it come through.
So seems the first thing is identifying the fluid... before you start replacing parts.
Brake fluid is typically clear in what I have used - but there are many different varieties now... I haven't cracked open the DOT 5 LV fluid my rig calls for yet - but have a bottle on board.
A leaking master cylinder can typically be noticed by pressing hard on the brake pedal while stopped... The pedal will stop at some point - then slowly start to descend again as it loses pressure. In bad cases the pedal will go to the floor.
Eventually you will see the loss of fluid in the reservoir as well.
Power steering fluid you may see the leak around the pump - and again fluid level will show eventually...
Antifreeze is many different colors depending on type used... This would run into the passenger compartment through the heater core. Usually has a 'sweet' smell associated with it - I wouldn't normally describe as oily though. Be sure to keep any pets away from this.
Once we can narrow down the fluid and its source - can determine what to replace.