In Mac OS8, there is apparently a setting somewhere which causes the
mouse pointer to automatically jump to the default button (e.g., "OK")
on any dialog box which pops up.  Where is this controlled from, and
how do I disable it?

Having written software that does specifically this, (
http://www.edenware.com/snap-to/ ) I can authoritatively say that
there is no such option in a standard install of Mac OS 8.

I can also point out that by far the most common cause of this
happening without Snap-To being installed is via an option in the
Kensington MouseWorks control panel, installed with Kensington mice.
If you have this software installed, I believe that the "Snap-To"
option is a simple toggleable menu item in the MouseWorks software.

I believe that the Logitech mouse software also contains such an
option.

In the unlikely case that you have none of the products installed, it
is possible that you have some other software installed with a
third-party mouse or pointing device, whose control panel you should
examine for such an option.

Dynamite!  You're absolutely right.  I hadn't been able to find a Mac
OS control ... nor a control in the Kensington MouseWorks panel.  But
I was looking in the latter's GUI, and hadn't noticed a nondescript
menu item called Options > Snap to Default.  That did the trick.

I love authoritative answers ... thanks again!