Is there a bus from Hong Kong Chungking Mansion to Saiwanho Building?

Yes, you can take bus no.110 at East Tsim Sha Tsui Station, Salisbury Road, get off at Tai Hong House, Tai Hong Street. Then walk southeast around 330 yards (300 meters) to find the destination. The whole way takes around 45 mins. It costs 9.3 HKD per person.

Thank you for your answer, it is very helpful. The fare 9.3 HKD it is octopus card fare? How about the fare for senior age? My parents is going with us, we are planning to purchase octopus card for senior age above 65.

No, it's not the octopus card fare.
When taking public bues like Kowloon Bus, Elder Octopus Card holders can pay the half-fare.
Besides, max 2 HKD fare applies to each journey when fare paid by Elder Octopus Card, otherwise half-fare concession applies.

Bus and fare from Saiwanho Building going to HK Museum of Coastal Defense? Chungking Mansion to Ocean Park? Mansion to Saiwanho Building fare by Octopus Card and elderly Octopus Card?

From Sai Wan Ho Building to Museum: take bus line 85 from Hoi An Street, Shau Kei Wan Road to HK Museum of Coastal Defence, Tung Hei Road. The octopus fare is HKD 3.7.

From Chungking Mansion to Ocean Park: take bus line 973 from New World Centre, Salisbury Road to Wong Chuk Hang San Wai, Wong Chuk Hang Road Station. The octopus fare is HKD 13.6.

From Chungking Mansion to Sai Wan Ho Building: take bus line 110 from East Tsim Sha Tsui Station, Salisbury Road to Tai Hong House, Tai Hong Street. The octopus fare is HKD 9.3.

Elderly octopus card holders pay half price. :)

thank you very much, your answer is very helpful