You Do Need power plug adapter while traveling from Serbia to Cocos (Keeling) Islands.

Here is Why You Need An Adapter

The outlet and plug type used in Cocos (Keeling) Islands is I and in Serbia are C and F. This is how they look like.

Serbia

In Serbia, outlets of type C and F are in use.

Type C

Type F

Cocos (Keeling) Islands

In Cocos (Keeling) Islands, outlets of type I are in use.

Type I