Learn Turkish: O kim? O bir mühendis. - Who is that? That is an engineer.

In Turkish, question words usually go in the exact same spot where the answer will be. Because the answer (mühendis) comes at the end of the sentence O bir mühendis, the question word kim (who) also goes at the end of O kim.

The translation says "Who is that?", but doesn't O mean "he" or "she"?

Yes, o means "he", "she", "it", and "that"! Turkish doesn't have gendered pronouns. Depending on the situation, O kim? could easily translate to "Who is he?" or "Who is she?". "Who is that?" is just a very natural English translation when you are pointing someone out or asking about someone new.

Where is the word for "is" in these sentences?

There isn't one! Turkish doesn't use a separate verb word for "to be" (am/is/are) in present-tense sentences like this. For "he/she/it/that", you simply put the subject (O) and the noun (kim or mühendis) together, and the "is" is naturally implied.

Do I always have to use bir when saying someone's profession?

Actually, it is more common to leave it out! Saying O mühendis is the most natural way to state someone's profession in Turkish. Including bir (a/an) is perfectly correct, but it can sometimes sound a bit like a direct translation from English.