I got the motor heads modified (think thats what you call it), timing belt and water pump replaced in March 2014. The last few months I've been having problems with my car over heating. It turns out it's the water pump, which was under warranty, so took it back to the mechanic shop and had it replaced again. Now about 1 month later, I hear a knocking in the engine, park the car and next day I check the water reservoir and there is oil mixed in with the water. The mechanic takes my car again, and says that it's due to "engine oil cooler is broken or damaged". So being a women, it makes sense, or what do you all think?
I suspect the part that the mechanic may have been referring to was the Oil Cooler. I'm not familiar with this specific vehicle but many engines employ an oil cooler which uses engine coolant to cool the oil. This takes in oil under pressure from the oil pump and feeds it back into the oil system, on it's coolant side, it takes coolant in from the water pump and feeds it back into the cooling system. If this is corroded it can allow the oil and water to mix inside it.
That said, oil in the coolant is usually an indication of a failed cylinder head gasket which is quite an involved and potentially expensive job to replace.
Your best bet is to have the engine compression tested (or leak down tested, a slightly different process but essentially checks for similar things) and only if this test (quick and cheap to do) confirms that the engine is healthy should you proceed with spending money changing things like the oil cooler.
Out of interest, is the oil in the coolant black or is it creamy like mayo? If it's a mayo like consistency this indicates it's been getting incredibly hot and you should get a compression or leak down test done as soon as possible.
It could be like what the mechanic said and would not be too expensive to fix You only need to change the radiator. Now you have had overheating, which is an engine destroyer, and you heard knocking sound, which does not sound too good. The engine "head" might have warped causing the oil to leak into coolant through the head gasket which is sandwiched between head and engine block. If this were the case, you are looking at thousands to change the cylinder head and head gasket. In the worst case, which is not uncommon, your engine internals might have be damaged and you need a new 2nd hand engine! So start by changing the radiator as per the mechanic, and see if it stops there. Never overheat engine. This is the engine destroyer/car buster.