Hi Yusuke, Both expressions describe a state of great happiness. Example: He was over the moon/on top of the world about his victory and told me it would inspire him to go further. "On top of the world" is used to describe one's great joy at achieving success or triumph in some endeavour. Example: She was on top of the world after clinching the top award for her art pieces.

No - both mean you are really happy about something:-) Note that we don't usually put 'the' before 'top' - 'on top of the world'

Yes, of course - everything is different. 'over the moon' is really pleased/delighted about something that happened (it makes you really happy) 'on top of the world' is enjoying your high position of success/happiness. They're used in different situations.