wrote:

If the OBS is set to the course, you can immediately discern whether
you are north or south of an eastbound course, for example. You can
do this by looking at a properly set OBS ring, and at which way the
needle is leaning.

Setting the OBS during a localizer approach is a good mnemonic. I do that.
It's also a good habit.

But it doesn't really impact *interpretation* of the CDI's display (unless
one forgets the course marked by the localizer, in which case the OBS
setting saves having to look down at the plate).

Using your example, the OBS doesn't do anything but remind you that the
course indicated by the localizer is eastbound. Useful, true, but I'd not
consider that a part of the interpretation process. I make this
distinction because, unless I forget the course, I never need to look at
the OBS to discern the meaning of the CDI.

- Andrew