how do i get to Ocean Park from Rambler Garden Hotel? taxi? rough estimate on the taxi fare please?

From your hotel, you first need take the shuttle bus to reach Tsing Yi Station where you can take the Tung Chung Line to reach Hong Kong Station. Upon reaching HK Station, you can walk to reach Central Station where you can find the bus 629 to take you to reach the Ocean Park.
If you travel alone, it is not suggested to take a taxi to reach the park. If you travel in a group, you may choose to take a taxi. I seldom traveled with taxi, because the taxi fare is too high in HK, so it is suggested to use the public taxi.

How much will be the fare if not taxi?

Within 20 HKD.