There are a couple of things that can cause oil to get into your coolant reservoir. A blown head gasket or a broken piston ring are the most common causes.
coolant fan
Coolant is used to keep an engine in a vehicle at a temperature safe for operation. If no coolant is in the reservoir, an engine will not start.
The radiator cap could be the cause of coolant reservoir overflow or backup. Have the cap checked to see if it is bad.
Either coolant is low or the sensor in the reservoir is stuck with dirt. Clean the reservoir with soap and water
If the coolant reservoir can not maintain the proper level of coolant, it is possible the engine could over heat when the level drops.
It probably has a leak. Here's something to check - the radiator has a tube coming out to the reservoir, so when it gets hot and expands the coolant goes into the reservoir and should go back into the radiator when it cools, but if your reservoir has a crack and the coolant leaks out, well it's gone and can't go back into the radiator - this would cause a lot of loss of coolant.
Do you have an oil cooler? I have heard that is a possible cause, as would be the head gaskets.
fan or thermostat not working
It can if it is stuck closed the engine overheats causing hot air ( steam ) to flow backwards to radiator cap which releases and blows steam pressure into recovery reservoir which starts blowing out the coolant in the reservoir making a puddle on ground.
old fuel filter
This is a indication of a blown head gasket,unless it has been over filled or overheated.If you are getting combustion in the cooling system you will see air coming up through the coolant in the reservoir,be careful it can blow hot coolant out without any warning.Most shops have a tool that will detect combustion gases in the coolant,it should be a inexpensive test.
No, this could mean you have a cracked head gasket and engine pressure has entered the coolant.