You already have an oil cooler (if your engine code is AHF, 110bhp). As someone pointed out it's underneath the oil filter and is cooled by the coolant system (oil to water cooler rather than oil to air).
You seem to be confusing the
problem somewhat. Your oil temperature isn't the cause of the overboost. It just so happens that your overboost is occurring when you have high oil temperatures, but it's not happening as a direct result of high oil temps. The oil thinning as it gets hotter means that the turbo is able to produce the maximum boost that the map is asking for, and this is too high for the MAP sensor to read, and hence the ecu reacts with a turbo shut down.
Your map is written to produce the boost that the turbo is providing, and the MAP sensor is unable to read the boost.
So, reduce the boost map to drop boost into the readable area of your existing MAP sensor, or fit a MAP sensor which is able to read the boost your turbo is set to produce.
Someone else asked, what grade oil are you using? Decent oil will
help with stability of oil at high temperatures. Remember the oil cooler? As it's coolant driven, what's the state of your coolant system, as this will affect your oil temps if it's sludging up or blocked (don't read the engine temp gauge, as these are notoriously vague in operation)