Whatever tire and wheel combination you’re planning, look at the tires first and find a wheel on the narrower end of the recommended off-road wheel width and on the wider end of the on-road handling range. Everything is a compromise of one thing or another.
To maintain rear clearance (distance from wheel to suspension) for every inch wider the wheel, the offset will have to change by half that amount. Offset is measured in millimeters and wheel width in inches, so for every 1 inch of wheel width added to the stock wheel, you subtract about 12-13mm from the stock offset.
So if you have a 7″ wide wheel with 55mm offset, an 8″ wide wheel would need about 42mm offset to maintain rear clearance.
Some people get wheels with an even lower offset to make their wheels stick out, but if you get big tires sometimes too low, an offset will cause friction. I have no experience with the 04 gen Outback so hopefully someone with that experience can chime in.
