I'm assuming that setting the Oregon to True, then the compass points to True North(internally corrects). Setting it to Magnetic then the Oregon points to magnetic North as a real compass?
I'va also got an Oregon 450. You select "true up" or "track up" for your Map under the "Map" category in the "Setup" menu. You select "True", "Magnetic", "Grid", or "Other" under the "Heading category in the "Setup " menu. In other words, the setup for your 450 compass is separate from the setup for your 450 map display (which does not have a "Magnetic up" option).
Quite a few years ago, I set up my installed chartplotter to "magnetic up" to keep it oriented in a manner consistent with my lliquid-filled pedestal compass. I thought it would be more intuitive that way. However, the chart display in magnetic mode tilted the legends in a staircase fashion, making the legends harder to read. After a while, i went back to the chartplotter being true north up and have kept it there ever since. I use this same system on my Oregon 450. The map is true north up and the compass is magnetic.
One reason I keep the chartplotter and the Oregon maps in true north up mode is that they are easier to relate to my paper charts. I keep the 450 compass in magnetic mode because it is easier to relate to my pedestal compass. One thought is that I don't want to confuse myself if I lose my electronics and fall back to old fashioned paper charts and old fashioned compass mode.