The writer pretends to oppose two mere antonyms or synomyms whereas in fact they have different meanings. That's a stylistic device aiming at overcoming some contradictions some tensions between seemingly opposite concepts.
THe paradox plays with logical propositions . Those propositions most of the time are not purely logical they stem from popular wisdom.
Paradoxs therefore aims at debunking sterotypes.
These are just clues to help you analyse paradoxes and oxymorons. have a try.. I 'll give you further details if needed.