Slight oil leak from above oil filter (oil cooler?)

Just above my oil filter is a bronze coloured metal thing (water to oil cooler?) This appears to have a very slow leak which drops down over the oil filter.
Apparently there is a nut above the oil filter that allows access to the cooler to replace an o-ring.
The workshop manual says that draining the oil is necessary (obviously), but also that all the coolant must be drained too.
Has anyone done replaced the o-ring in the oil cooler before? And if so, is it straightforward?
Kim
Apparently there is a nut above the oil filter that allows access to the cooler to replace an o-ring.
The workshop manual says that draining the oil is necessary (obviously), but also that all the coolant must be drained too.
Has anyone done replaced the o-ring in the oil cooler before? And if so, is it straightforward?

Kim