I wached the video and everything seems in place in my filter.As Colin had mentioned, the O ring is placed in between the motor and the filter(the part where the motor is attached/joined to the filter) to prevent water from leaking out.
Here are some links about the C4 spare parts for your understanding.
Replacement Parts - Fluval C Series Instruction Manual [Page 3]
Fluval C Series Manual Online: Replacement Parts. To re-order parts for your Fluval C Power Filter, please refer to the parts list below. D B A F I H G J L C2 ITEM A Filter Case A20265 B A20268 Biological Trickle Chamber with cover C Chemical Basket A20271 D Filter Cover A20274 E...www.manualslib.com
I have had no problems with water leaking out so I guess I have the o ring and I just didn't notice