I've been there recently.
There is a convenient package from KCRC
- Ocean park ticket + free KCR east-rail ride + shuttle-bus from Hunghom Station (10:55am to / 6pm return, daily). The whole journey takes <50 mins.
Domestic package costs HK$198 (adult) / HK$99 (kids)
Can buy the package in any KCR station including the TaiPo one.
Note: Ocean Park opening hours: 10am - 6pm.
Details:
http://www.oceanpark.com.hk/eng/main/index.html