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.

おかしなこと、おかしいこと。 It is same thing for Japanese. we say それは、おかしい。 But we cannot say それは、おかしな。 おかしなの後には、必ず名詞がきます。 おかしな　is used with a noun;

All Chinese and Japanese questions asked by the members who registered in 2008 are tricky questions. This is a Japanese grammar question, it has been just asked few days ago. If your question is answer under my name, it is foreseeable, it is the incorrect answer. So, I send my answer to the Japanese friends, let them publish the answer for me. If you don't get an answer from the native Japanese, then it is a prefixed question and the answer has been reserved for those who control the answer behind the scene.