I heard only the top of the top can land an oral surgery residency. how true is this?

You're Accurate: Post graduate programs are competitive and since the spaces are limited, the programs can choose the candidates they want. This usually translates to students who graduated at the top of their class.

All specialties: As opposed to medicine where the majority of medical school graduates enter a specialty residency, the opposite is true of dentistry. Less than 10% of dental school graduates are accepted into specialty programs, and they tend to be at the top of their class. In our post-graduate orthodontic program we tend to take students in the top 2.5% of their graduating class.