That's the entire function of the oil cooler, it exchanges heat between the engine oil and coolant. Oil flows through it to the filter and then back through it to the engine block, and coolant flows through it via a pair of rubber hoses. The hoses are flexible, so you can unbolt the cooler from the block and move it around some (plenty for replacing it's Oring). One of those hoses goes to a steel line that crosses in front of the oil pan, this can be unbolted from the engine to gain even more flexibility without opening the cooling system. I'm not sure if it'll be enough to really be beneficial in accessing the oil switch.
In 2008, Subaru switched coolant types. I've seen newer cars with conventional green in them, but I certainly wouldn't mix them, at which point a gallon of Subaru stuff typically makes more sense then flushing the system to replace with conventional green fluid.
Adding fluid isn't any different. I think that setup still has a conventional radiator cap/overflow bottle.