Complex sentence: It is uncommon to present the BN (O) Passport to enter Hong Kong, however, in such cases, the person's Hong Kong Identity Card number could be retrieved from his or her BN (O) passport.
Split into simpler sentences: It is uncommon to present the BN (O) Passport to enter Hong Kong. In such case, however, the person's Hong Kong Identity Card number could be retrieved from his or her BN (O) passport.