Is there bus from Ocean park back to the train station after 4pm so we can get to hong kong airport

Yes, there is. I remember the last bus of bus no. 620 leaves at 21:00 (for Admiralty before, but the last stop has been changed to Tin Hau for the current situation). After visiting, you can take the bus to Tin Hau. Then take Island Line at this MTR station to Central. Walk to HK station through an underpass to take Airport Express to HKIA.