An oxymoron is a term used to indicate a phrase that contains conflicting ideas. 

A copy is a duplicate of the original. Therefore, 'a original copy' is an oxymoron as the original cannot be a copy. 
'Crowd' means a large gathering. Therefore, 'a small crowd' is an oxymoron.
A secret is something that is supposed to be known by others. Therefore, 'open secret' is an oxymoron as well.

All the given options are oxymorons. Therefore, option D is the right answer.