My definition of $\omega-$categorical: a theory $T$ is $\omega-$categorical if any two countable models of $T$ are isomorphic. In particular, if $T$ doesn't have a countable model, then $T$ is $\omega-$categorical.
I am solving some exercises from Hodges shorter model theory, and while solving Ex 6.3.1, I wanted to figure out the following:
I either want to find an example or prove that such example cannot exist, of the following:
Find an example of a language $L$ and an infinite $L$ structure $M$, such that there is a finite tuple $\vec{m}\in M$ such that precisely one of the following two theories is $\omega-$categorical: $Th(M)$ and $Th(M,\vec{m})$.
If $L$ is countable, then by Ryll-Nardzewski, using the fact that "$S_n(Th(M))$ is finite for all $n$ iff $S_n(Th(M,\vec{m}))$ is finite for all $n$", we can see that such an example cannot exist.
Thanks for any help!