でなく, じゃなく, and ではなく are all conjugated. I looked up my dictionary and found that でない, じゃない, and ではない are all correct collocations. The dictionary doesn't make any judgment on formality between any two of the three. My judgment is this:
(more formal) ではない > でない ≧ じゃない
Generally, でない should be more formal than じゃない; however, as でない sounds much lazier than ではない, it could sometimes sound as lazy as じゃない, at least to my ears; so lazy sometimes that I was not sure whether or not でない was really acceptable grammatically although it actually is.
Specifically about おれだけでなく and おれだけじゃなく, some natives may say the former is more formal. But to my ears there's not much difference, because おれだけでなく gives me an impression that は is missing no matter whether dictionaries have labeled ではない and でない are both correct.
If someone is still interested in the topic.
Google gives more results on ~だけでなくて versus ~だけではなくて.
I'm not a native, so that's just my view on this issue.
First, I saw でない in attributive position pretty often (especially in technical documentation), unlike predicative position where it's usually ではない (じゃない).
Like, in バカでない人 or 有効でない文字 or something like that.
Second - and I think this is more important - は may imply contrastive meaning.
親切でない - not kind (simple statement)
親切ではない - as for kindness - he is not (though he may have some other good sides)
It's very subtle.
But in case of だけ we already have similar meaning in a sentence.
おれでない - not me
おれではない - not me (but others, may be)
おれだけでない - not only me (others too)
おれだけではない - not only me (others too; and others, may be)
So, to avoid that similar (though not identical) semantics people avoid using は sometimes. But since it's so subtle both variants are valid.