Antifreeze mixed with your oil will destroy the internal engine bearings and all moving components. The antifreeze will cause a lack of lubrication and all metal to metal surfaces will wear to the point of failure.
There are tubes that run from the block into the head that carry oil and coolant. This is to cool and lubricate the head and valvetrain. In order to keep the coolant and oil in their correct tubes, there is a gasket between the cylinder head and block. It has holes cut into it that match the cooland/oil passages and allows it to pass while sealing around it. If your engine has run hot (the usual cause for a blown headgasket) it warps the head an leaves a gap between the head/block. When this happens the pressures inside the cylinder escape through the weaker gasket material, and usually break the seal on the oil/coolant passages.
So now you have the oil and coolant passages where they can mix together. The main problem is that you have coolant in the engine oil. This is not good, since all the internal parts of the engine are metal. I would pull the head off and replace the gasket ASAP. Stop driving this car until you get this repaired.
HEadgasket failure, internal oil cooler leak.
antifreeze leaking into the oil pan
Make sure you use the red stuff as the green antifreeze tends to rot the oil cooler and causes other problems too. Do not mix them either.
most likely a blown head gasket.
an oil leak
There SHOULD only be oil inside the valve covers. If they leak coolant you have a serious problem.
The only reason is a leak. Usually small. The place to start is: If it smells like oil or antifreeze ...
Because it 's leaking- OIL, ANTIFREEZE, or vacume leak
cracked head or bad gasket
You may have blown a head gasket, check you oil to see if the antifreeze is leaking into there.
check your oil to see if you have coolant in it. if you do your intake gasket needs changed.