Can oral sex cause thrush?

Yes it is possible. Yes, it's possible to pass a yeast infection to a partner during unprotected oral sex. Symptoms include an irritated or itchy mouth. If a known or suspected yeast infection is present in either partner, it is best to avoid oral sex until treated properly.
No. If an adult has oral or vaginal thrush, the doctor will look for the usual causes of thrush, regardless of whether or not she does oral sex. The candida that overgrows to cause thrush normally lives in people's mouths and vaginas, in small amounts. Having oral sex doesn't make the candida grow more.