As you may know, おかしい is one of i-adj, which modify noun like おかしいこと、おかしいひと(weird person).
There are also na-adj, which modify noun like しずかな人(quiet person), しずかなところ(quiet place).
However, おかしな is none of the above. It's just an independent noun modifier(I don't know what scholars call it in English, but anyway). It never change its form, and it can only work to modify a noun.
Other examples for this kind of words are:
おおきな(big), ちいさな(small), あの(that)
So, the な of おかしな has nothing to do with ナadj, and the meaning is basically same as おかしい.