Oil in cooling system?

Have had everything replaced on the cooling system due to a starting from a crack in the radiator. The car never overheated from what the temperature gauge shown but I noticed the leak from the radiator. I ended up replacing the radiator, hoses, thermostat, and water pump at my local mechanic I use. Get my vehicle back and notice I have to fill my coolant reservoir multiple times during small trips around town. Take it back and they do a pressure test to the cooling system to locate leak to find nothing mentioning a little air was in the system saying pressure was good. Get vehicle back to again having to add coolant then noticed a leak at the big hose at bottom of radiator leaking due to a loose clamp. I also noticed the fan is not properly seated in bottom retainer clips on the two bottom clips that secure fan to radiator. Take back to mechanic and now they say oil is leaking into my cooling system. What can cause this to happen and why could they not find the leak in the cooling system during the pressure test they performed? Is there any way they could be the cause of the oil in my cooling system? If they can be responsible for it, can I make them fix the problem due to it being their fault?