Is possible to visit the Hong Kong giant buddha,victoria peak and ocean park in one day.

It will be a challenge to finish these places in one day. Well, in the morning around 08:30, you could take metro for about 40 minutes from the hotel to Tung Chung station and then take bus 23 to get to the Giant Buddha. Around 12:00, you should back to Tung Chung and then take the metro (40 minutes) to Admiralty station. At the Exit B, you could take bus 629 to Ocean Park. When the tour end, take bus back to Admiralty and hire a taxi driving to the Peak Tram station. The Hong Kong nightview could be enjoyed at the Peak.
Ps: if you have children together, it is not suggested to take this itinerary in one day.