[texhax] Forcing size changes in exponents

[Michael Barr]

I have often been criticized by posters on these boards for "not using LaTeX".  Why I should care is a real question.  But here is something I really tried to do in LaTeX and failed, although it is almost trivial using the deprecated command \rm:

\def\op{^{\rm op}}
 $A\op$
 $A^{A\op}$
 
When I tried do it using \textrm, the size did not change.  So I added \scriptsize.  I expected the first line to be correct, I expected the second would not.  Instead I got a missing $ error.  I tried sprinkling some $ signs into the defiinition and the first one worked (although I don't see why), but the second still caused an error.

So how can you do this in pure LaTeX?

Michael