YOU SAID:
Kim Namjoon is my oppa.
INTO JAPANESE
キムナムジュンは私の反対です。
BACK INTO ENGLISH
Kim Nam Jun is my opposite.
INTO JAPANESE
キム・ナムジュンは私の反対です。
BACK INTO ENGLISH
Kim Nam Jung is against me.
INTO JAPANESE
キムナムジョンは私に反対です。
BACK INTO ENGLISH
Kim Nam Jung is against me.
That didn't even make that much sense in English.