You, asshole. あなたはアスホールですね

I believe shouting 「サノバビッチ!」(Son-of-a-bitch) or 「ガッデム!」 (God, dammit) is not that uncommon among young Japanese men. My PC knew both in katakana, which means there are the word entries in the dictionary. When saying one of these, you should completely drop an English accent to give it a funny effect.

Those are first exposed to Japanese in manga in '70s to '80s. And, at somepoint, people start to use heavily-Japanese-accented English phrased like these. オーマイゴッド is another fairly common phrase you hear in a casual Japanese conversation. People use it jokingly.

I love it when an American says オーマイゴッド with a heavy and flat Japanese accent with a dead plain face. It is so funny, and most Japanese would get it as a joke.

Since we can infer that:

1. the American person knows that we know the English phrase.
2. the American person knows that we sort of make fun of it as very 外人ぽいセリフ (being very foreign).
3. despite the said somewhat racist ridicule, the American person had the nerve to laugh it away with us.

Saying オーマイゴッド as a native English speaker shows his/her deep understanding of the two languages and cultures behide them. It also shows your openess to different cultures. So, I recommend those phrases as a joke!

「あなたはアスホールですね」

would sound funny. It would be a lot funnier if said with a serious look. You probably want to frown but stay calm, looking into the eyes of the person you are saying it to. Oh, I should use this someday myself...

Phonetically, in modern spoken Japanese, we unconciously drop some vowels in phrases like:

タクシー (ta-ku-shi-i) --> (ta-k-shi-i)
〜です (de-su) --> (de-s)

This trend has been catching on. So, to get the most out of this katakana English joke, you may want to emphasize vowels in every syllable.