Really? from what I've learnt I would've thought that the katakana would be 'オーカミ' being as the elongated vowel in katakana is usually written as a dash rather than another vowel character. Even so, Ōkami is a Japanese word, so I don't think it would ever really be written in katakana, would it?
As for the hiragana, I feel I made an error - isn't the elongated 'o' sound in hiragana usually made using the character 'う', making the hiragana 'おうかみ'?
And as for the kanji, I think it's either, the difference being that '狼' means 'wolf' whereas '大神' means 'great god'. Depends on which one you mean, but they're both pronounced the same though, I think.
Then again, I'm a noob at Japanese, so I'm not sure of these things.